3.5. The GHCi Debugger
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## 3.5. The GHCi Debugger

GHCi contains a simple imperative-style debugger in which you can stop a running computation in order to examine the values of variables. The debugger is integrated into GHCi, and is turned on by default: no flags are required to enable the debugging facilities. There is one major restriction: breakpoints and single-stepping are only available in interpreted modules; compiled code is invisible to the debugger[5].

The debugger provides the following:

- The ability to set a breakpoint on a function definition or expression in the program. When the function is called, or the expression evaluated, GHCi suspends execution and returns to the prompt, where you can inspect the values of local variables before continuing with the execution.

- Execution can be single-stepped: the evaluator will suspend execution approximately after every reduction, allowing local variables to be inspected. This is equivalent to setting a breakpoint at every point in the program.

- Execution can take place in tracing mode, in which the evaluator remembers each evaluation step as it happens, but doesn't suspend execution until an actual breakpoint is reached. When this happens, the history of evaluation steps can be inspected.

- Exceptions (e.g. pattern matching failure and `error`) can be treated as breakpoints, to help locate the source of an exception in the program.

There is currently no support for obtaining a “stack trace”, but the tracing and history features provide a useful second-best, which will often be enough to establish the context of an error. For instance, it is possible to break automatically when an exception is thrown, even if it is thrown from within compiled code (see Section 3.5.6, “Debugging exceptions”).

### 3.5.1. Breakpoints and inspecting variables

Let's use quicksort as a running example. Here's the code:

    qsort [] = []
    qsort (a:as) = qsort left ++ [a] ++ qsort right
      where (left,right) = (filter (<=a) as, filter (>a) as)

    main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])

First, load the module into GHCi:

    Prelude> :l qsort.hs
    [1 of 1] Compiling Main             ( qsort.hs, interpreted )
    Ok, modules loaded: Main.
    *Main>

Now, let's set a breakpoint on the right-hand-side of the second equation of qsort:

    *Main> :break 2
    Breakpoint 0 activated at qsort.hs:2:15-46
    *Main>

The command `:break 2` sets a breakpoint on line 2 of the most recently-loaded module, in this case `qsort.hs`. Specifically, it picks the leftmost complete subexpression on that line on which to set the breakpoint, which in this case is the expression `(qsort left ++ [a] ++ qsort right)`.

Now, we run the program:

    *Main> main
    Stopped at qsort.hs:2:15-46
    _result :: [a]
    a :: a
    left :: [a]
    right :: [a]
    [qsort.hs:2:15-46] *Main>

Execution has stopped at the breakpoint. The prompt has changed to indicate that we are currently stopped at a breakpoint, and the location: `[qsort.hs:2:15-46]`. To further clarify the location, we can use the `:list` command:

    [qsort.hs:2:15-46] *Main> :list
    1  qsort [] = []
    2  qsort (a:as) = qsort left ++ [a] ++ qsort right
    3    where (left,right) = (filter (<=a) as, filter (>a) as)

The `:list` command lists the source code around the current breakpoint. If your output device supports it, then GHCi will highlight the active subexpression in bold.

GHCi has provided bindings for the free variables[6] of the expression on which the breakpoint was placed (`a`, `left`, `right`), and additionally a binding for the result of the expression (`_result`). These variables are just like other variables that you might define in GHCi; you can use them in expressions that you type at the prompt, you can ask for their types with `:type`, and so on. There is one important difference though: these variables may only have partial types. For example, if we try to display the value of `left`:

    [qsort.hs:2:15-46] *Main> left

    <interactive>:1:0:
        Ambiguous type variable `a' in the constraint:
          `Show a' arising from a use of `print' at <interactive>:1:0-3
        Cannot resolve unknown runtime types: a
        Use :print or :force to determine these types

This is because `qsort` is a polymorphic function, and because GHCi does not carry type information at runtime, it cannot determine the runtime types of free variables that involve type variables. Hence, when you ask to display `left` at the prompt, GHCi can't figure out which instance of `Show` to use, so it emits the type error above.

Fortunately, the debugger includes a generic printing command, `:print`, which can inspect the actual runtime value of a variable and attempt to reconstruct its type. If we try it on `left`:

    [qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
    [qsort.hs:2:15-46] *Main> :print left
    left = (_t1::[a])

This isn't particularly enlightening. What happened is that `left` is bound to an unevaluated computation (a suspension, or thunk), and `:print` does not force any evaluation. The idea is that `:print` can be used to inspect values at a breakpoint without any unfortunate side effects. It won't force any evaluation, which could cause the program to give a different answer than it would normally, and hence it won't cause any exceptions to be raised, infinite loops, or further breakpoints to be triggered (see Section 3.5.3, “Nested breakpoints”). Rather than forcing thunks, `:print` binds each thunk to a fresh variable beginning with an underscore, in this case `_t1`.

The flag `-fprint-evld-with-show` instructs `:print` to reuse available `Show` instances when possible. This happens only when the contents of the variable being inspected are completely evaluated.

If we aren't concerned about preserving the evaluatedness of a variable, we can use `:force` instead of `:print`. The `:force` command behaves exactly like `:print`, except that it forces the evaluation of any thunks it encounters:

    [qsort.hs:2:15-46] *Main> :force left
    left = [4,0,3,1]

Now, since `:force` has inspected the runtime value of `left`, it has reconstructed its type. We can see the results of this type reconstruction:

    [qsort.hs:2:15-46] *Main> :show bindings
    _result :: [Integer]
    a :: Integer
    left :: [Integer]
    right :: [Integer]
    _t1 :: [Integer]

Not only do we now know the type of `left`, but all the other partial types have also been resolved. So we can ask for the value of `a`, for example:

    [qsort.hs:2:15-46] *Main> a
    8

You might find it useful to use Haskell's `seq` function to evaluate individual thunks rather than evaluating the whole expression with `:force`. For example:

    [qsort.hs:2:15-46] *Main> :print right
    right = (_t1::[Integer])
    [qsort.hs:2:15-46] *Main> seq _t1 ()
    ()
    [qsort.hs:2:15-46] *Main> :print right
    right = 23 : (_t2::[Integer])

We evaluated only the `_t1` thunk, revealing the head of the list, and the tail is another thunk now bound to `_t2`. The `seq` function is a little inconvenient to use here, so you might want to use `:def` to make a nicer interface (left as an exercise for the reader!).

Finally, we can continue the current execution:

    [qsort.hs:2:15-46] *Main> :continue
    Stopped at qsort.hs:2:15-46
    _result :: [a]
    a :: a
    left :: [a]
    right :: [a]
    [qsort.hs:2:15-46] *Main>

The execution continued at the point it previously stopped, and has now stopped at the breakpoint for a second time.

#### 3.5.1.1. Setting breakpoints

Breakpoints can be set in various ways. Perhaps the easiest way to set a breakpoint is to name a top-level function:

       :break identifier

Where `identifier` names any top-level function in an interpreted module currently loaded into GHCi (qualified names may be used). The breakpoint will be set on the body of the function, when it is fully applied but before any pattern matching has taken place.

Breakpoints can also be set by line (and optionally column) number:

       :break line
       :break line column
       :break module line
       :break module line column

When a breakpoint is set on a particular line, GHCi sets the breakpoint on the leftmost subexpression that begins and ends on that line. If two complete subexpressions start at the same column, the longest one is picked. If there is no complete subexpression on the line, then the leftmost expression starting on the line is picked, and failing that the rightmost expression that partially or completely covers the line.

When a breakpoint is set on a particular line and column, GHCi picks the smallest subexpression that encloses that location on which to set the breakpoint. Note: GHC considers the TAB character to have a width of 1, wherever it occurs; in other words it counts characters, rather than columns. This matches what some editors do, and doesn't match others. The best advice is to avoid tab characters in your source code altogether (see `-fwarn-tabs` in Section 5.7, “Warnings and sanity-checking”).

If the module is omitted, then the most recently-loaded module is used.

Not all subexpressions are potential breakpoint locations. Single variables are typically not considered to be breakpoint locations (unless the variable is the right-hand-side of a function definition, lambda, or case alternative). The rule of thumb is that all redexes are breakpoint locations, together with the bodies of functions, lambdas, case alternatives and binding statements. There is normally no breakpoint on a let expression, but there will always be a breakpoint on its body, because we are usually interested in inspecting the values of the variables bound by the let.

#### 3.5.1.2. Listing and deleting breakpoints

The list of breakpoints currently enabled can be displayed using `:show breaks`:

    *Main> :show breaks
    [0] Main qsort.hs:1:11-12
    [1] Main qsort.hs:2:15-46

To delete a breakpoint, use the `:delete` command with the number given in the output from `:show breaks`:

    *Main> :delete 0
    *Main> :show breaks
    [1] Main qsort.hs:2:15-46

To delete all breakpoints at once, use `:delete *`.

### 3.5.2. Single-stepping

Single-stepping is a great way to visualise the execution of your program, and it is also a useful tool for identifying the source of a bug. GHCi offers two variants of stepping. Use `:step` to enable all the breakpoints in the program, and execute until the next breakpoint is reached. Use `:steplocal` to limit the set of enabled breakpoints to those in the current top level function. Similarly, use `:stepmodule` to single step only on breakpoints contained in the current module. For example:

    *Main> :step main
    Stopped at qsort.hs:5:7-47
    _result :: IO ()

The command `:step `expr`` begins the evaluation of `expr` in single-stepping mode. If `expr` is omitted, then it single-steps from the current breakpoint. `:stepover` works similarly.

The `:list` command is particularly useful when single-stepping, to see where you currently are:

    [qsort.hs:5:7-47] *Main> :list
    4
    5  main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
    6
    [qsort.hs:5:7-47] *Main>

In fact, GHCi provides a way to run a command when a breakpoint is hit, so we can make it automatically do `:list`:

    [qsort.hs:5:7-47] *Main> :set stop :list
    [qsort.hs:5:7-47] *Main> :step
    Stopped at qsort.hs:5:14-46
    _result :: [Integer]
    4
    5  main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
    6
    [qsort.hs:5:14-46] *Main>

### 3.5.3. Nested breakpoints

When GHCi is stopped at a breakpoint, and an expression entered at the prompt triggers a second breakpoint, the new breakpoint becomes the “current” one, and the old one is saved on a stack. An arbitrary number of breakpoint contexts can be built up in this way. For example:

    [qsort.hs:2:15-46] *Main> :st qsort [1,3]
    Stopped at qsort.hs:(1,0)-(3,55)
    _result :: [a]
    ... [qsort.hs:(1,0)-(3,55)] *Main>

While stopped at the breakpoint on line 2 that we set earlier, we started a new evaluation with `:step qsort [1,3]`. This new evaluation stopped after one step (at the definition of `qsort`). The prompt has changed, now prefixed with `...`, to indicate that there are saved breakpoints beyond the current one. To see the stack of contexts, use `:show context`:

    ... [qsort.hs:(1,0)-(3,55)] *Main> :show context
    --> main
      Stopped at qsort.hs:2:15-46
    --> qsort [1,3]
      Stopped at qsort.hs:(1,0)-(3,55)
    ... [qsort.hs:(1,0)-(3,55)] *Main>

To abandon the current evaluation, use `:abandon`:

    ... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
    [qsort.hs:2:15-46] *Main> :abandon
    *Main>

### 3.5.4. The `_result` variable

When stopped at a breakpoint or single-step, GHCi binds the variable `_result` to the value of the currently active expression. The value of `_result` is presumably not available yet, because we stopped its evaluation, but it can be forced: if the type is known and showable, then just entering `_result` at the prompt will show it. However, there's one caveat to doing this: evaluating `_result` will be likely to trigger further breakpoints, starting with the breakpoint we are currently stopped at (if we stopped at a real breakpoint, rather than due to `:step`). So it will probably be necessary to issue a `:continue` immediately when evaluating `_result`. Alternatively, you can use `:force` which ignores breakpoints.

### 3.5.5. Tracing and history

A question that we often want to ask when debugging a program is “how did I get here?”. Traditional imperative debuggers usually provide some kind of stack-tracing feature that lets you see the stack of active function calls (sometimes called the “lexical call stack”), describing a path through the code to the current location. Unfortunately this is hard to provide in Haskell, because execution proceeds on a demand-driven basis, rather than a depth-first basis as in strict languages. The “stack“ in GHC's execution engine bears little resemblance to the lexical call stack. Ideally GHCi would maintain a separate lexical call stack in addition to the dynamic call stack, and in fact this is exactly what our profiling system does (Chapter 6, Profiling), and what some other Haskell debuggers do. For the time being, however, GHCi doesn't maintain a lexical call stack (there are some technical challenges to be overcome). Instead, we provide a way to backtrack from a breakpoint to previous evaluation steps: essentially this is like single-stepping backwards, and should in many cases provide enough information to answer the “how did I get here?” question.

To use tracing, evaluate an expression with the `:trace` command. For example, if we set a breakpoint on the base case of `qsort`:

    *Main> :list qsort
    1  qsort [] = []
    2  qsort (a:as) = qsort left ++ [a] ++ qsort right
    3    where (left,right) = (filter (<=a) as, filter (>a) as)
    4
    *Main> :b 1
    Breakpoint 1 activated at qsort.hs:1:11-12
    *Main>

and then run a small `qsort` with tracing:

    *Main> :trace qsort [3,2,1]
    Stopped at qsort.hs:1:11-12
    _result :: [a]
    [qsort.hs:1:11-12] *Main>

We can now inspect the history of evaluation steps:

    [qsort.hs:1:11-12] *Main> :hist
    -1  : qsort.hs:3:24-38
    -2  : qsort.hs:3:23-55
    -3  : qsort.hs:(1,0)-(3,55)
    -4  : qsort.hs:2:15-24
    -5  : qsort.hs:2:15-46
    -6  : qsort.hs:3:24-38
    -7  : qsort.hs:3:23-55
    -8  : qsort.hs:(1,0)-(3,55)
    -9  : qsort.hs:2:15-24
    -10 : qsort.hs:2:15-46
    -11 : qsort.hs:3:24-38
    -12 : qsort.hs:3:23-55
    -13 : qsort.hs:(1,0)-(3,55)
    -14 : qsort.hs:2:15-24
    -15 : qsort.hs:2:15-46
    -16 : qsort.hs:(1,0)-(3,55)
    <end of history>

To examine one of the steps in the history, use `:back`:

    [qsort.hs:1:11-12] *Main> :back
    Logged breakpoint at qsort.hs:3:24-38
    _result :: [a]
    as :: [a]
    a :: a
    [-1: qsort.hs:3:24-38] *Main>

Note that the local variables at each step in the history have been preserved, and can be examined as usual. Also note that the prompt has changed to indicate that we're currently examining the first step in the history: `-1`. The command `:forward` can be used to traverse forward in the history.

The `:trace` command can be used with or without an expression. When used without an expression, tracing begins from the current breakpoint, just like `:step`.

The history is only available when using `:trace`; the reason for this is we found that logging each breakpoint in the history cuts performance by a factor of 2 or more. GHCi remembers the last 50 steps in the history (perhaps in the future we'll make this configurable).

### 3.5.6. Debugging exceptions

Another common question that comes up when debugging is “where did this exception come from?”. Exceptions such as those raised by `error` or `head []` have no context information attached to them. Finding which particular call to `head` in your program resulted in the error can be a painstaking process, usually involving `Debug.Trace.trace`, or compiling with profiling and using `+RTS -xc` (see Section 6.3, “Time and allocation profiling”).

The GHCi debugger offers a way to hopefully shed some light on these errors quickly and without modifying or recompiling the source code. One way would be to set a breakpoint on the location in the source code that throws the exception, and then use `:trace` and `:history` to establish the context. However, `head` is in a library and we can't set a breakpoint on it directly. For this reason, GHCi provides the flags `-fbreak-on-exception` which causes the evaluator to stop when an exception is thrown, and `-fbreak-on-error`, which works similarly but stops only on uncaught exceptions. When stopping at an exception, GHCi will act just as it does when a breakpoint is hit, with the deviation that it will not show you any source code location. Due to this, these commands are only really useful in conjunction with `:trace`, in order to log the steps leading up to the exception. For example:

    *Main> :set -fbreak-on-exception
    *Main> :trace qsort ("abc" ++ undefined)
    "Stopped at <exception thrown>
    _exception :: e
    [<exception thrown>] *Main> :hist
    -1  : qsort.hs:3:24-38
    -2  : qsort.hs:3:23-55
    -3  : qsort.hs:(1,0)-(3,55)
    -4  : qsort.hs:2:15-24
    -5  : qsort.hs:2:15-46
    -6  : qsort.hs:(1,0)-(3,55)
    <end of history>
    [<exception thrown>] *Main> :back
    Logged breakpoint at qsort.hs:3:24-38
    _result :: [a]
    as :: [a]
    a :: a
    [-1: qsort.hs:3:24-38] *Main> :force as
    *** Exception: Prelude.undefined
    [-1: qsort.hs:3:24-38] *Main> :print as
    as = 'b' : 'c' : (_t1::[Char])

The exception itself is bound to a new variable, `_exception`.

Breaking on exceptions is particularly useful for finding out what your program was doing when it was in an infinite loop. Just hit Control-C, and examine the history to find out what was going on.

### 3.5.7. Example: inspecting functions

It is possible to use the debugger to examine function values. When we are at a breakpoint and a function is in scope, the debugger cannot show you the source code for it; however, it is possible to get some information by applying it to some arguments and observing the result.

The process is slightly complicated when the binding is polymorphic. We show the process by means of an example. To keep things simple, we will use the well known `map` function:

    import Prelude hiding (map)

    map :: (a->b) -> [a] -> [b]
    map f [] = []
    map f (x:xs) = f x : map f xs

We set a breakpoint on `map`, and call it.

    *Main> :break 5
    Breakpoint 0 activated at  map.hs:5:15-28
    *Main> map Just [1..5]
    Stopped at map.hs:(4,0)-(5,12)
    _result :: [b]
    x :: a
    f :: a -> b
    xs :: [a]

GHCi tells us that, among other bindings, `f` is in scope. However, its type is not fully known yet, and thus it is not possible to apply it to any arguments. Nevertheless, observe that the type of its first argument is the same as the type of `x`, and its result type is shared with `_result`.

As we demonstrated earlier (Section 3.5.1, “Breakpoints and inspecting variables”), the debugger has some intelligence built-in to update the type of `f` whenever the types of `x` or `_result` are discovered. So what we do in this scenario is force `x` a bit, in order to recover both its type and the argument part of `f`.

    *Main> seq x ()
    *Main> :print x
    x = 1

We can check now that as expected, the type of `x` has been reconstructed, and with it the type of `f` has been too:

    *Main> :t x
    x :: Integer
    *Main> :t f
    f :: Integer -> b

From here, we can apply f to any argument of type Integer and observe the results.

    *Main> let b = f 10
    *Main> :t b
    b :: b
    *Main> b
    <interactive>:1:0:
        Ambiguous type variable `b' in the constraint:
          `Show b' arising from a use of `print' at <interactive>:1:0
    *Main> :p b
    b = (_t2::a)
    *Main> seq b ()
    ()
    *Main> :t b
    b :: a
    *Main> :p b
    b = Just 10
    *Main> :t b
    b :: Maybe Integer
    *Main> :t f
    f :: Integer -> Maybe Integer
    *Main> f 20
    Just 20
    *Main> map f [1..5]
    [Just 1, Just 2, Just 3, Just 4, Just 5]

In the first application of `f`, we had to do some more type reconstruction in order to recover the result type of `f`. But after that, we are free to use `f` normally.

### 3.5.8. Limitations

- When stopped at a breakpoint, if you try to evaluate a variable that is already under evaluation, the second evaluation will hang. The reason is that GHC knows the variable is under evaluation, so the new evaluation just waits for the result before continuing, but of course this isn't going to happen because the first evaluation is stopped at a breakpoint. Control-C can interrupt the hung evaluation and return to the prompt.

  The most common way this can happen is when you're evaluating a CAF (e.g. main), stop at a breakpoint, and ask for the value of the CAF at the prompt again.

- Implicit parameters (see Section 8.7.2, “Implicit parameters”) are only available at the scope of a breakpoint if there is an explicit type signature.

[5] Note that packages only contain compiled code, so debugging a package requires finding its source and loading that directly.

[6] We originally provided bindings for all variables in scope, rather than just the free variables of the expression, but found that this affected performance considerably, hence the current restriction to just the free variables.

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