# Hat -- The Haskell Tracer
Version 2.00
Users' Manual

### The ART Team

### 14 June 2002

## 1 Introduction

Hat is a source-level tracer for Haskell (the Haskell Tracer). It is a tool that gives the user access to otherwise invisible information about a computation. Thus Hat helps locating errors in programs. However, it is also useful for understanding how a correct program works, especially for program maintenance. Hence we avoid the popular name ``debugger''. Note that a profiler, which gives access to information about the time or space behaviour of a computation, is also a kind of tracer. However, Hat is not intended for that purpose. Hat measures neither time nor space usage.
Conventional tracers (debuggers) for imperative languages allow the user to step through the computation, stop at given points and examine variable contents. This tracing method is unsuitable for a lazy functional language such as Haskell, because its evaluation order is complex, function arguments are usually unwieldy large unevaluated expressions and generally computation details do not match the user's high-level view of functions mapping values to values.
Hat is an offline tracer: First the specially compiled program runs as normal, except that additionally a trace is written to file. Second, after the computation has terminated, the trace is viewed with a number of browsing tools.
Hat can be used for computations that terminate normally, that terminate with an error message or that are interrupted by the programmer (because they do not terminate).
The trace consists of high-level information about the computation. It describes each reduction, that is, the replacements of an instance of a left-hand side of an equation by an instance of its right-hand side, and the relation of the reduction to other reductions.
Because the trace describes the whole computation, it is huge. Hence the programmer uses tools to selectively view the fragments of the trace that are of interest. Currently Hat includes four tools -- hat-observe, hat-trail, hat-detect, and hat-stack -- for that purpose. Each tool shows fragments of the computation in a particular way, highlighting a specific aspect.

## 2 Obtaining the Trace of a Computation

To obtain a trace of a computation of a program, the program has to be compiled specially, using `hmake` with either `nhc98` or `ghc`, and then run.
Compile the program using `hmake` and the `-hat` option; you may want to choose your compiler as well, e.g. `hmake -ghc -hat`.
What `hmake` does is this: all the modules of the program are transformed to tracing versions with the pre-processor `hat-trans`. This preprocessor generates a new module (prefixed with the letter `T') for each original module. The generated modules are compiled and linked using an ordinary compiler with the extra option `-package hat`. The `hat` package contains interface files and a link-library that are needed by the transformed program.
You can invoke `hat-trans` and the compiler manually if you wish, but `hat-trans` generates and reads its own special kind of module interface files (`.hx` files) and therefore modules must be transformed in the same dependency order as normal compilation. Hence, it is much easier simply to let `hmake` do all the work.
Note that the `hat-trans` preprocessor does not insert the complete file paths of the original source modules into the generated modules. The trace viewers assume that the source modules are in the same directory as the executable.

### 2.1 Compilation with nhc98

Tracing makes computations use more heap space. As a rough rule of thumb, traced computations require 3 times as much heap space as untraced ones. However, because traced computations allocate (and discard) much memory, it is useful to choose an even larger heap size to reduce garbage collection time. The preset default heap size for an untraced program compiled by `nhc98` is 400KB; you will probably want to increase this to at least 2MB. For example, you can set the heap size at compile (link) time with `+CTS -H10m -CTS` or for a specific computation with `+RTS -H10m -RTS` to a ten megabyte heap.

### 2.2 Computation

The traced computation behaves exactly like the untraced one, except that it is slower (currently about 100 times slower in nhc98, 200 times slower in ghc), and additionally writes a trace to file.
If it seems that the computation is stuck in a loop, then force halting by keying an interrupt (usually `Ctrl-C`). After termination of the computation (normal termination or caused by error or interrupt) you can explore the trace with any of the programs described in the following sections.
The computation of a program name creates the trace files name`.hat`, name`.hat.bridge` and name`.hat.output`. The latter is a copy of the whole output of the computation. The first is the actual trace. It can easily grow to several hundred megabytes. To improve the runtime of the traced computation you should create the trace file on a local disc, not on a file system mounted over a network. The trace files are always created in the same directory as the executable program.

### 2.3 Trusting

Hat enables you to trace a computation without recording every reduction. You can trust the function definitions of a module. Then the calls of trusted functions from trusted functions are not recorded in the trace.
Note that a call of an untrusted function from a trusted function is possible, because an untrusted function can be passed to a trusted higher-order function. These calls are recorded in the trace.
For example, you may call the trusted function `map` with an untrusted function `prime`: `map prime [2,4]`. If this call is from an untrusted function, then the reduction of `map prime [2,4]` is recorded in the trace, but not the reductions of the recursive calls `map prime [4]` and `map prime []`. However, the reductions of `prime 2` and `prime 4` are recorded, because `prime` is untrusted.
You should trust modules in whose computations you are not interested. Trusting is desirable for the following reasons:

- to keep the size of the trace file smaller (main point)
  - to save file space
  - to avoid unnecessary detail when viewing the trace
- to reduce the runtime of the traced program (slightly)

If you want to trust a module, then compile it for tracing as normal but with the extra option `-trusted`. (A plain object file compiled without any tracing option cannot be used.) By default the Prelude and the standard libraries are trusted.

## 3 Viewing a Trace

Although each tool gives a different view on the trace, they all have some properties in common.

### 3.1 Arguments in Most Evaluated Form

The tools show function arguments in evaluated form, more precisely: as far evaluated as the arguments are at the end of the computation. For example, although in a computation the unevaluated expression `(map (+5) [1,2])` might be passed to the function `length`, the tools show the function application as `length [1+5,2+5]` or `length [_,_]` if the list elements are unevaluated.

### 3.2 Special Expressions

##### Unevaluated expressions

Tools do not usually show non-value subexpressions. The underscore `_` represents these unevaluated expressions. (The `uneval' option can be set interactively if you wish to replace underscores with the full representation of the unevaluated expression.)

##### l-abstractions

A l-abstraction, as for example `\xs-> xs ++ xs`, is represented simply by `(\..)`.

##### The undefined value ^

If the computation is aborted because of a run-time error or interruption by the user, then evaluation of a redex may have begun, but not yet resulted in a value. We call the result of such a redex undefined and denote it by ^ (`_|_` in ASCII form).
A typical case where we obtain ^ is when in order to compute the value of a redex the value of the redex itself is needed. The occurrence of such a situation is called a black hole. The following example causes a black hole:

>     a = b + 1
>     b = a + 1

>     main = print a

When the program is run, it aborts with an error message saying that a black hole has been detected. The trace of the computation contains several ^'s.

##### Trusted Expressions

The symbol `{?}` is used to represent an expression that was not recorded in the trace, because it was trusted.

### 3.3 Combination of Viewing Tools

Each tool gives a unique view of a computation. These views are complementary and it is productive to use them together. From each of the three tools hat-observe, hat-trail and hat-detect you can at any time change to any of the other two tools, starting there at exactly the point of the trace at which you left the other tool. So after using one tool to track a bug to a certain point you can change to another tool to continue the search or confirm your suspicion.

### 3.4 The Running Example

The following faulty program is used as example in the description of most viewing tools:

>     main = let xs :: [Int]
>                xs = [4*2, 5`div`0, 5+6]
>            in  print (head xs, last' xs)

>     last' (x:xs) = last' xs
>     last' [x] = x

## 4 Hat-Observe

Hat-observe enables you to observe the value of top-level variables, that is, functions and constants. Hat-observe shows all reductions of a variable that occurred in the traced computation. Thus for a function it shows all the arguments with which the function was called during the computation together with the respective results.
It is possible to use hat-observe in batch-mode from the command line, but the main form of use is as an interactive tool. The interactive mode provides more comprehensive facilities for filtering the output than batch mode.

### 4.1 Starting & Exiting

To start hat-observe as an interactive tool, simply enter

> `hat-observe` prog`[.hat]`

at the command line, where prog is the name of the traced program.

### 4.2 The Help Menu

Enter `:h` (`:help`) to obtain a short overview of the commands understood by hat-observe. All commands begin with a ``:`', and can be shortened to any prefix of the full name.

### 4.3 Observing for Beginners: Using the Wizard

If you use hat-observe for the first time, you might want to start by using the observation wizard. Simply enter the command `:observe` with no other arguments. The tool will then ask questions about the reductions you are interested in. Eventually, it will show the resulting query and start the observation. This way you can quickly learn what queries look like.

### 4.4 Making Simple Observations

Observations of a function are made with the `:observe` command, or for simplicity, just by entering the name of the function at the prompt. For instance, enter `:observe` f, or simply f, to obtain all reductions of f.
To avoid redundant output, equivalent reductions of the identifier are omitted in the display (`:set unique`). You can change this behaviour in order to see all reductions, even identical ones (`:set all`). In future there will also be an option to see only the most general reductions. A reduction of an identifier is considered more general than another if all its arguments on the left-hand-side are less defined (due to lazy evaluation) and/or if its result on the right-hand-side is more fully defined.

### 4.5 Exploring What to Observe

If you forgot the correct spelling of a function identifier you want to observe or you do not know the program well, you may want to see a list of all function identifiers which can be observed. With the `:info` command you can browse the list of all top-level function identifiers which were used during the computation, and how many times they were used.

### 4.6 Filtering Reductions

Even when only unique reductions are shown, some observations may still result in an excessively large number of displayed equations. You only want to see those reductions in which you are particularly interested. There are several ways to decrease the number of reductions shown.

#### 4.6.1 Non-Recursive Mode

Hat-observe can omit recursive calls of the given function. If all the top-most calls of a function are correct, then all its recursive calls within the function itself are likely to be correct as well. If there are any erroneous recursive calls, their incorrect behaviour at least had no effect on the result of the top most calls. To omit recursive calls of a function, the `:set recursive off` command may be used. To see recursive calls again, use `:set recursive on`.

#### 4.6.2 Observing Calls from a Specific Context

Another way to restrict the number of reductions being observed is by observing only calls made from within a specific calling function. If you are interested in all calls of `map` from the function `myMapper`, try `:observe map in myMapper`.

#### 4.6.3 Specifying Reductions with a Pattern

You can significantly reduce the number of observed applications by observing only reductions that are instances of a given pattern. With a pattern you can specify in which reductions you are particularly interested.
You can enter a pattern for the whole equation or any prefix of it. A pattern for an equation consists of a pattern for the left-hand-side followed by a `=` and a pattern for the result. The `=` and result pattern may be omitted, as may any of the trailing argument patterns.
If you wish to skip one argument in the pattern, use an underscore. An underscore `_` in a pattern matches any expression, value, or unevaluated. The bottom symbol `_|_` may also be used in patterns, and matches only unevaluated things.
Examples:

- To see all applications of `map` where its first argument is `foo`, enter `:observe map foo`. However, to see all applications of `map` where its second argument is `foo`, enter `:observe map _ foo`.
- To see all applications of `filter` using first argument `odd` and resulting in an empty list, enter `:observe filter odd _ = []`.

Infix patterns are also supported, although the fixity and priority of the operator is not necessarily known, so always use explicit parentheses around such patterns.
Sugared syntax for strings and lists is supported, e.g. `"Hello world!"` for a string and `[1,2,42]` etc. for lists.

#### 4.6.4 Combination of Filters

Of course, all methods previously described can be mixed with each other, as in the following examples.
`:observe map _ [1,2,3] in myMapper`
`:observe filter even (: 1 _) = _|_ in myFunction`
`:observe fibiterative _ _ = 0`

### 4.7 Verbose Modes

There are several modes determining the relative verbosity of the output. `:set uneval on` shows unevaluated expressions in full, rather than abbreviating them to underscores. `:set strSugar off` turns off string sugaring, and `:set listSugar off` turns off sugaring for other kinds of list: in both cases, the effect is to reveal the explicit cons and nil structures.

### 4.8 Browsing a List of Reductions

After successfully submitting a query in any of the described ways, the tool searches the given trace file. Depending on the size of the file and the number of reductions found, the search may take a considerable time. Progress will be indicated during the scan of the file. After the scan of the file, additional time might be spent on filtering the most general reductions matching the given pattern.
The first n (default 10) observed reductions are then displayed. More reductions can be displayed by pressing the `RETURN` key. The system indicates the availability of additional equations by prompting with `--more-->` instead of the usual command prompt. If more equations are available but you do not wish to see them, typing anything except the plain RETURN key will cause you to leave the equation display mode and go back to the normal prompt.
The number of equations displayed per group can be altered by using the `:set group` n command. The default is 10 reductions at a time. The reductions are numbered -- this is to facilitate selection of an equation for use within the other hat tools.
Attention: because hat-observe uses lazy evaluation to determine the list of reductions, there may be a delay during which more reductions are determined.

### 4.9 Display of Large Expressions

Sometimes expressions may contain very large data structures which clutter the display. In order to cope with them the cutoff depth of the display can be adjusted. This cutoff value determines the nesting depth to which nested sub-expressions are printed: any subexpression beyond this depth is shown as a dark square. The cutoff depth is adjusted using the command `:set cutoff` n.
In certain circumstances, you simply want to increase or decrease the cutoff by a small amount. There are `shortcut' commands `:+` n and `:-` n to increase or decrease the cutoff by n respectively. If n is omitted, then it is assumed to be one.
A data structure may be infinite. Because an infinite data structure is the result of a finite computation, it must contain a cycle. The following example demonstrates how such a cycle is shown.

>     cyclic = 1:2:3:4:cyclic
>     main = putStrLn (show (take 5 cyclic))

If you observe `cyclic`, then you obtain

>     cyclic = (cyc1 where cyc1 = 1:2:3:4:cyc1)

### 4.10 Invoking other Viewing Tools

You may eventually find an erroneous reduction. There are several ways in which you can proceed at this point.
The first way is to start observing functions used in the definition body of the erroneous function. You will need to check the source code for functions which might have caused the wrong result. If you suspect a function f to have caused the incorrect behaviour of g, it is a good idea to try `:observe` f `in` g.
Alternatively, you have the choice to use `hat-trail` on a reduction you have observed. Use the command `:trail` `n` to start a separate instance of `hat-trail` for equation number n.

### 4.11 Quick reference to commands

All the commands that are available in hat-observe are summarised in the following table.

    ---------------------------------------------------------------------------
     <query>            observe the named function/pattern
     <RETURN>           show more observations (if available)
     :observe <query>   observe the named function/pattern
     :info              see a list of all observable functions
     :detect <n>        start hat-detect on equation <n>
     :trail <n>         start hat-trail browser on equation <n>
     :source <n>        show the source application for equation <n>
     :Source <n>        show the source definition for identifier in eqn <n>
     :set               show all current mode settings
     :set <flag>        change one mode setting
       <flag> can be: uneval [on|off]      show unevaluated expressions in full
                      strSugar [on|off]    sugar character strings
                      listSugar [on|off]   sugar lists
                      recursive [on|off]   show recursive calls
                      [all|unique]         show all equations or only unique
                      group <n>            number of equations listed per page
                      cutoff <n>           cut-off depth for deeply nested exprs
     :+[n]              short-cut to increase cutoff depth by <n> (default 1)
     :-[n]              short-cut to decrease cutoff depth by <n> (default 1)
     :resize            detect new window size for pretty-printing
     :help              show this help text
     :quit              quit
    ---------------------------------------------------------------------------

## 5 Hat-Trail

Hat-trail is an interactive tool that enables you to explore a computation backwards, starting at the program output or an error message (with which the computation aborted). This is particularly useful for locating an error. You start at the observed faulty behaviour and work backwards towards the source of the error.
Every reduction replaces an instance of the left-hand side of a program equation by an instance of its right-hand side. The instance of the left-hand side ``creates'' the instance of the right-hand side and is therefore called its parent.
Using the symbol ¬ to represent the relationship ``comes from'', (i.e. the arrow points from parent to child) here is an illustrative list showing the parent of every subexpression from the example in Section 3.

> the error message ¬ `last' []`
> `last' []` ¬ `last' (_:[])`
> `last' (_:[])` ¬ `last' (_:_:[])`
> `last' (_:_:[])` ¬ `last' (8:_:_:[])`
> `last' (8:_:_:[])` ¬ `main`
> `8` ¬ `4*2`
> `4*2` ¬ `xs`

Every subexpression (if it is not a top-level constant such as `main`) has a parent. In the example the parent of `(8:_:_:[])` is `xs`. The parent of each subexpression in an expression can be different from the parent of the expression itself.

### 5.1 Starting & Exiting

Start hat-trail by entering

> `hat-trail` prog`[.hat]`

at the command line, where prog is the name of the program (the extension `.hat` is optional).
You can quit this browser at any time by typing the command :quit.

### 5.2 The Help Menu

The :help command offers short explanations of the main features of hat-trail, similar to the quick reference of Section 5.7.

### 5.3 Basic Exploration of a Trace

The browser window mainly consists of two panes:

- The program output (and error) pane.
  Here you can select a part of the program output (or an error message, if there was one), to show its parent redex in the trace pane for further exploration.
- The trail pane.
  This is the most important pane. In it you explore the trace. With the cursor keys you request information about different parts of the trace. Coloured highlighting is used to show the current and previous selections.

You can pop up a further, very important, window on demand:

- The source code window.
  Here part of the source code of the traced program is shown. In the trail pane, you can ask to see a specific point in the source code (e.g. where exactly a function in a particular expression was applied), and the cursor in the source code window is placed at the relevant site in the appropriate source file. This is not an editor window, just a viewer -- type the `q' or `x' key in the window to close it.

#### 5.3.1 The program output (and error) pane

Any output (or error message) produced by the traced program is shown in the top pane. The output is divided into sections; there is one section of output for each output action performed by the program. You select a section of the output with the cursor keys: left/up and right/down. The selected section is shown with a coloured highlight. If the output is very large, only a portion of it is displayed at a time -- moving left/up at the top of the screen, or right/down at the bottom, `pages' through the output. Press the Return key to start exploring the parent redex for the selected section in the lower trail pane.

#### 5.3.2 The trail pane

Within the trail pane, the display shows a simple `stack' of parent expressions, one per line. Each expression line is the parent of the highlighted subexpression on the line before it. Within a line, you can navigate to a subexpression using the cursor keys by one of two methods, described below. Pressing the Return key asks for the parent of the currently selected subexpression, and it is shown on a new line. Pressing the Delete or Backspace key removes the current line and goes back to the previous selection in the stack.

##### Selecting a subexpression in the trail pane

There are two methods of navigating within an expression to highlight a specific subexpression. The simplest method just uses the right and left cursor keys. Repeatedly pressing the right cursor key follows a pre-order traversal of the underlying expression tree. Thus, first an application is highlighted, then the function part, then each argument, and so on recursively depth-first. The left cursor key follows the reverse order.
Alternatively, you can navigate by explicit levels within the tree. The up cursor key moves outwards from a subexpression to the surrounding expression. The down key moves inwards from an application to the function position. The `[' and `]' keys move left and right amongst subexpressions at the same level, for instance between a function and its arguments.

##### Folding away part of an expression

When you are looking at a large expression, it is sometimes difficult to see its gross structure because of all the detail. At these times, it is helpful to be able to shrink certain subexpressions to hide the detail. This is represented in the display as a small dark box.
You can explicitly shrink any selected subexpression to a dark box, or expand a selected box to its full representation, with the `-' and `+' keys.
Like in the other tools, there is a standard cutoff depth for deeply nested expressions. The automatically cutoff expression is denoted by the same dark box as a manually hidden expression. Use the `:set cutoff` n command to change the cutoff depth, or the shortcuts `:+` n and `:-` n to increase and decrease the cutoff depth -- if n is omitted, the cutoff is increased or decreased by one.

#### 5.3.3 The source code window

Most functions and constants are used at more than one position in the program. Hence, when viewing an expression in hat-trail, it can be very helpful to know exactly which application site is under examination. There are a number of direct links to the source code available.
First, the filename, line, and column number of the currently selected application or value is always visible in a status line at the top of the trail pane. You can use this to help you navigate within your preferred editor or viewer.
Secondly, the command `:source` pops up a simple viewer with the cursor sitting directly over the application site in the relevant file. The command can be abbreviated to `:s`, and you may find that this is often quicker and more convenient than using an external editor or viewer.
Thirdly, if you want to look at the definition of the selected function or constant rather than its individual application site, the command `:Source` again pops up the simple viewer, this time with the cursor on the definition line.

#### 5.3.4 Special syntax

We have already mentioned that a dark box represents a subexpression that has been hidden from display, either due to automatic cutoff of deep nesting, or by explicit request of the user.
There are a number of other special syntactic representations in the display.

##### Lists

A list where some elements are undefined or unevaluated is displayed as a sequence of nested applications of the normal list constructor `(:)`. However, fully evaluated lists are displayed using the more compact syntactic sugar of square brackets with elements separated by commas. Where the end of list is cutoff (due to the normal cutoff depth parameter), this can look slight ambiguous. Is the dark box the final element of the list, or does it represent a larger tail of the list? To find out, you need to expand the dark box.

##### Strings

Fully evaluated character strings are displayed differently again. A string is usually shown using the Haskell lexical convention of double quotes, for example `"Hi"`. In this representation, the string is treated as a single atomic unit for navigation. If you really want to select a singe character or substring, then you must turn string sugaring off (`:set strSugar off`) in order to navigate within the explicit list.

##### Control-flow constructs

The control-flow in a function is determined by conditional expressions (`if` `then` `else`), `case` expressions and guards. It is often desirable to see why a certain branch was taken in such a control-flow construct. For example, the problem in a function definition might not be that it computes a wrong return value, but that a test is erroneous which makes it select a branch that returns the wrong value.
A control-flow expression of this nature is shown in the trail as the value of the guard, condition or case discriminant, placed to the right of the expression within which it belongs and separated from it by a bar and a highlighted keyword, e.g. `| if False` or `| case EQ`.
Strictly speaking, the expression to the left of the bar is the parent of the expression on the right, but the tool displays them together for clarity since the guard, condition, or case makes most sense when understood in the context of its parent.
For example, in the program

>     abs x | x < 0 = -x
>           | otherwise = x

>     main = print (abs 42)

the parent of the result value `42` is

>     abs 42 | False | True

This redex display states that the second branch in the definition of `abs` was taken. The last guard was evaluated to `True` whereas the previous guard was evaluated to `False`. You may ask for the parent of `False` and learn that it was created by the redex `42 < 0`.

##### Trusting

Section 2.3 describes trusting of modules as a means to obtain a smaller trace.
In general the result of a trusted function may be an unevaluated expression from within the trusted function. Such an expression is shown with the symbol `{?}`. It cannot be expanded like a dark box representing a cutoff expression, but it does have a parent. For example, for the program

>     main = print (take 5 (from 1))

the parent of the result value `[1,2,3,4,5]` is

>     take 5 (1:2:3:4:5:{?})

The parent of `{?}` is `from 1`, as for the whole expression `(1:2:3:4:5:{?})`.

##### Unevaluated expressions

Unevaluated expressions are shown by default with the underscore symbol (`_`). Show you wish to see these expressions in full, you should switch on the ``show-me-unevaluated-expressions'' option with the command `:set uneval on`.

#### 5.3.5 Pattern bindings

A program equation with a single variable or a pattern with variables on the left hand side is a pattern binding. The parent of a variable defined by a pattern binding is not the redex that called it, but the redex on whose right-hand-side the pattern binding occurs. Hence variables defined by top-level pattern bindings (i.e. constants) do not have parents.
So usually the parent of an expression is the function call that would have led to the evaluation of the expression if eager evaluation were used. However, this relation breaks down for pattern bindings.

### 5.4 Advanced Exploration of a Trace

#### 5.4.1 Shared expressions

When you select a subexpression in the trail pane, sometimes not only this expression is highlighted but also some other occurrences of the subexpression. The reason is that the marked occurrences are shared. That is, they are not just equal, but they actually share the same space in memory. This operational observation can often help you to understand a computation.

### 5.5 Invoking other Viewing Tools

It is possible to invoke hat-observe and hat-detect immediately from hat-trail.

- The command `:observe` launches a new window with the hat-observe tool, which immediately searches for all applications of the currently selected function throughout the computation.
- The command `:location` launches a new window with the hat-observe tool, which immediately searches for applications of the currently selected function, but only at the same source location as the current selection. This is useful to narrow down your exploration to a specific site of interest.
- The command `:detect` launches a new window with the hat-detect tool, restricting the debugging algorithm solely to the currently selected equation and all its dependents.
- The command `:trail` starts a new window with a fresh instance of the hat-trail tool starting with the current selection. This can be useful if there are several redex trails you wish to explore and compare side-by-side.

### 5.6 Some practical advice

- First-time users of hat-trail tend to quickly unfold large parts of the trace and thus clutter the screen and get lost. Think well, before you demand to see another parent. It is seldom useful to follow a long sequence of parents for whole redexes. Do not forget that you can ask for the parent of any subexpression. Choose the subexpression that interests you carefully. When locating an error, a wrong subexpression of an argument is a good candidate for further enquiry.
  In our experience usually less than 10 parents need to be viewed to locate an error, even in large programs.
- Use the links to the source as described in Section 5.3.3. The trail display is designed to be concise, so the source viewer gives valuable context information.
- Avoid l-abstractions in your program. Informative function names are very helpful for tracing.

### 5.7 Quick reference to commands

All the commands that are available in hat-trail are summarised in the following table.

    ----------------------------------------------------------------------------
     cursor keys   movement within current expression
     [ and ] keys  movement within current expression
     RETURN        show parent expression of selected expression
     =             show final result of the whole expression line
     BACKSPACE     remove most recently-added expression/equation
     -/+           shrink/expand a cutoff expression
     ^L            repaint the display if it gets corrupted
     ^R            repaint the display after resizing the window
     :source       look at the source-code application of this expression
     :Source       look at the source-code definition of current function
     :observe      use hat-observe to find all applications of this function
     :location     use hat-observe to find all applications at this call site
     :trail        start a fresh hat-trail with the current expression
     :detect       use hat-detect to debug the current expression
     :set          show all current mode settings
     :set <flag>   change one mode setting
     :set <flag>   change one mode setting
       <flag> can be: uneval [on|off]      show unevaluated expressions in full
                      strSugar [on|off]    sugar character strings
                      listSugar [on|off]   sugar lists with [,,,]
                      equations [on|off]   show equations, not just redexes
                      cutoff <n>           cut-off depth for deeply nested exprs
     :+[n]         shortcut to increase cutoff depth
     :-[n]         shortcut to decrease cutoff depth
     :help  :?     show this help text
     :quit         quit
    ----------------------------------------------------------------------------

## 6 Hat-Detect

For various reasons, hat-detect is not currently available in Hat 2.00. It will re-appear in a future release.
Hat-detect is an interactive tool that enables you to locate semi-automatically an error in a program by answering a sequence of yes/no questions. Each question concerns a reduction. You have to answer yes, if the reduction is correct with respect to your intentions, and no otherwise. After a number of questions hat-detect states which reduction is the cause of the observed faulty behaviour -- that is, which function definition is incorrect.

### 6.1 Limitations

At the moment hat-detect does not handle IO actions properly. It can only handle computations that perform a single primitive output action such as `putStr` or `print`. Monadic binding operators (or do-notation) and input actions such as `read` lead to confusion.
Hence the recommended usage of hat-detect is to first use hat-observe to locate an erroneous reduction that does not involve IO and then to invoke hat-detect for this reduction as described in Section 4.10.
Also, currently hat-detect can only be used for computations that produce faulty output, not for computations that abort with an error message or are interrupted (in the latter cases hat-detect may indicate a wrong error location).

### 6.2 Starting & Exiting

Start hat-detect by entering

> `hat-detect` prog`[.hat]`

where prog is the name of the traced program.
To exit hat-detect enter `:quit` or `:q`.

### 6.3 The Help Menu

Enter `:help` to obtain a short overview of the commands understood by hat-detect.

### 6.4 Basic Functionality

Consider the following program:

>     insert x [] = [x]
>     insert x (y:ys)
>       | x > y = x : insert x ys
>       | otherwise = x : y : ys

>     sort xs = foldr insert [] xs

>     main = print (sort [3,2,1])

It produces the faulty output `[3,3,3]` instead of the intended output `[1,2,3]`.
The following is an example session with hat-detect for the computation. The y/n answers are given by the user:

>     1  main = IO (print (3:3:3:[]))   ? n
>     2  sort (3:2:1:[]) = 3:3:3:[]   ? n
>     3  insert 1 [] = 1:[]   ? y
>     4  insert 2 (1:[]) = 2:2:[]   ? n
>     5  insert 2 [] = 2:[]   ? y

>     Error located!
>     Bug found: "insert 2 (1:[]) = 2:2:[]"

The first question of the session asks if the reduction of `main` is correct. Hat-detect indicates that `main` is reduced to an IO action, and shows the action. The answer is obviously no. Further answers from the user show that the third and the fifth reductions are correct, whereas the second and fourth are not.
Note that hat-detect does not ask about any reductions of `foldr` here, mainly because it is trusted.
After the answer to the fifth question hat-detect can determine the location of the error. The equation that is used to reduce the redex `insert 2 (1:[])` is wrong. Indeed, on the right-hand side of the guard `x > y` (viz: `2 > 1`) the result should be `y : insert x ys`.

#### 6.4.1 Postponing an Answer

If you are not sure about the answer to a question you can answer ?n or ?y. If you answer ?n, then hat-detect proceeds as if the answer had been no. But if it cannot locate an error in one of the child reductions, then it will later ask you the question again. Answering ?y will postpone the question as well, but hat-detect will proceed as if the answer hat been yes. If it cannot locate an error in one of its brother reductions, then it will ask you the question again.

#### 6.4.2 Unevaluated Subexpressions

Reductions may contain underscores `_` that represent unevaluated subexpressions. A question with an underscore on the left-hand side of the reduction has to be read as ``is the reduction correct for any value at this position?'' and a question with an underscore on the right-hand side should be read as ``is the reduction correct for some value at this position?''. If there are several underscores in a reduction the values at these positions need not be the same.

### 6.5 Algorithmic Debugging

Hat-detect is based on the idea of algorithmic/declarative debugging. The reductions of a computation are related by a tree structure. The reduction of `main` is the root of the tree. The children of a reduction of a function application are all those reductions that reduce expressions occurring on the right-hand side of the definition of the function.
If a question about a reduction is answered with no, then the next question concerns the reduction of a child node. However, if the answer is yes, then the next question will be about a sibling or a remaining node closer to the root.
An error is located when a node is found such that its reduction is incorrect but the reductions of all its children are correct. That reduction is the source of the error.

### 6.6 Advanced Features

#### 6.6.1 Single stepping

Hat-detect can be used rather similarly to a conventional debugger. So the input no means ``step into current function call'' and the input yes means ``go on to next function call''. Note that this single stepping is not with respect to the lazy evaluation order actually used in the computation, but with respect to an eager evaluation order that ``magically'' skips over the evaluation of expressions that are not needed in the remaining computation.

#### 6.6.2 Showing unevaluated subexpressions

By default hat-detect shows unevaluated subexpressions just as underscores `_`. For answering a question these unevaluated subexpressions are irrelevant anyway. However, by entering the `:set verbose on` command you can switch to verbose mode which shows these unevaluated subexpressions in full. Use `:set verbose off` to switch the verbose mode off again.

#### 6.6.3 Going back to a question

The questions are numbered. By entering a number n you can go back to any previous question numbered n. When you do this, the answers to all intervening questions are deleted.

#### 6.6.4 Trusting

Hat-detect does not ask any question about the reductions of functions that are trusted as described in Section 2.3. However, you can trust further functions and thus avoid questions about them. By entering `:trust` instead of `y` when being asked about a specific reduction of a function you trust this function. By entering `:untrust` you stop trusting all these functions again.

#### 6.6.5 Memoisation

By default hat-detect memoises all answers you gave. So, although the same reduction may be performed several times in a computation, hat-detect will only ask once about it. Hat-detect even avoids asking a question, if a more general question (containing more unevaluated expressions) has been answered before.
You can turn memoisation on/off with the command `:set memoize on` or `:set memoize off`.

#### 6.6.6 Invoking other Viewing Tools

##### Observing a function

When being asked about a specific reduction of a function you can enter `:observe` to observe the function. The hat-observe tool will appear in a new window, showing all applications of the given function. This interface to hat-observe is particularly useful, if you are not sure whether to trust a function for Algorithmic Debugging. By observing all applications of the function you can decide whether the function can indeed be trusted or not. If you find an erroneous reduction in the observation, you can select it and in turn start a new Algorithmic Debugging session for this reduction.

##### Tracing arguments

When `hat-detect` asks you about the reduction of an application, which obviously has a wrong argument, you should consider using `hat-trail` to investigate where this argument came from. By answering a question with `:trail` the Redex Trail browser is launched immediately from the Algorithmic Debugger.

### 6.7 Quick reference to commands

All the commands that are available in hat-detect are summarised in the following table.

    ---------------------------------------------------------------------------
     y   or  yes      you believe the equation is ok
     n   or  no       you believe the equation is wrong
     ?y  or  y?       you are not sure (but try ok for now)
     ?n  or  n?       you are not sure (but try wrong for now)
     <n>              go back to question <n>
     :set             show all current mode settings
     :set <flag>      change one mode setting
          <flag> can be: memoise [on|off]:  never ask the same question again
                         verbose [on|off]:  show unevaluated exprs in full
                         cutoff <n>:        set subexpression cutoff depth
     :observe         start hat-observe on the current function
     :trail           start hat-trail on the current equation
     :trust           trust all applications of the current function
     :untrust         untrust ALL functions which were previously trusted
     :help            show this help text
     :quit            quit
    ---------------------------------------------------------------------------

## 7 Hat-Stack

For aborted computations, that is computations that terminated with an error message or were interrupted, hat-stack shows in which function call the computation was aborted. It does so by showing a virtual stack of function calls (redexes). So every function call on the stack caused the function call above it. The evaluation of the top stack element caused the error or during its evaluation the computation was interrupted. The shown stack is virtual, because it does not correspond to the actual runtime stack. The actual runtime stack enables lazy evaluation whereas the virtual stack corresponds to a stack that would be used for eager (strict) evaluation.

### 7.1 Usage

To use hat-stack enter

> `hat-stack` programname

where programname is the name of the traced program.

### 7.2 Example

Here is an example output:

    Program terminated with error:
        "No match in pattern."
    Virtual stack trace:
        (last' [])                     (Example.hs: line-6/col-16)
        (last' (5+6:[]))               (Example.hs: line-6/col-16)
        (last' ((div 5 0):5+6:[]))     (Example.hs: line-6/col-16)
        (last' (8:(div 5 0):5+6:[]))   (Example.hs: line-4/col-27)
        main                           (Example.hs: line-2/col-1)

### 7.3 Further Information

Hat-trail can also show this virtual stack. Hat-stack is a simple tool that enables you to obtain the stack directly. The description of hat-trail contains more details about the relationships between the stack elements.

## 8 Limitations of Functionality

Although Hat can trace nearly any Haskell 98 program, some program constructs are still only supported in a restricted way. See the Hat web page for further limitations and bugs.

### 8.1 List Comprehensions

List comprehensions are desugared by Hat, that is, their implementation in terms of higher-order list functions such as `foldr` is traced.

### 8.2 Labelled Fields (records)

Expressions with field labels (records) are desugared by Hat. So viewing tools show field names only as selectors but never together with the arguments of a data constructor. An update using field labels is shown as a `case` expression.

### 8.3 Strictness Flags

Strictness flags in data type definitions are ignored by Hat and hence lose their effect.

> This document was translated from LATEX by HEVEA .