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Chapter 14
Support Facilities

## 14.1 Eval and Load

`(eval``` `expr``)` evaluates the S-expression `expr` in the current namespace.34 (See section 8 and section 7.7.1.5 for more information about namespaces.)

`(load``` `file-path``)` evaluates each expression in the specified file using ``eval``.35 The return value from ``load`` is the value of the last expression from the loaded file (or void if the file contains no expressions). If `file-path` is a relative pathname, then it is resolved to an absolute pathname using the current directory. Before the first expression of `file-path` is evaluated, the current ``load-relative`` directory (the value of the `current-load-relative-directory` parameter; see section 7.7.1.6) is set to the absolute pathname of the directory containing `file-path`; after the last expression in `file-path` is evaluated (or when the load is aborted), the ``load-relative`` directory is restored to its pre-``load`` value.

`(load-relative``` `file-path``)` is like ``load``, but when `file-path` is a relative pathname, it is resolved to an absolute pathname using the current ``load-relative`` directory rather than the current directory. If the current ``load-relative`` directory is `#f`, then ``load-relative`` is the same as ``load``.

`(load/use-compiled``` `file-path``)` is like ``load-relative``, but ``load/use-compiled`` also checks for `.zo` files (usually produced with ``compile-file``; see Chapter mz:compilefile in PLT MzLib: Libraries Manual) and `.so` (Unix and Mac OS) or `.dll` (Windows) files.36 The check for a compiled file occurs whenever `file-path` ends with a dotted extension of three characters or less (e.g., `.ss` or `.scm`) and when a `compiled` subdirectory exists in the same directory as `file-path`. A `.zo` version of the file is loaded if it exists directly in the `compiled` subdirectory. An `.so` or `.dll` version of the file is loaded if it exists within a `native` subdirectory of the `compiled` directory, in a deeper subdirectory as named by ``system-library-subpath``. A compiled file is loaded only if its modification date is not older than the date for `file-path`. If both `.zo` and `.so` or `.dll` files are available, the `.so` or `.dll` file is used.

Multiple files can be combined into a single `.so` or `.dll` file by creating a special dynamic extension `_loader.so` or `_loader.dll`. When such a extension is present where a normal `.so` or `.dll` would be loaded, then the `_loader` extension is first loaded. The result returned by `_loader` must be a procedure that accepts a symbol. This procedure will be called with a symbol matching the base part of `file-path` (without the directory path part of the name and without the filename extension), and the result must be two values; if `#f` is returned as the first result, then ``load/use-compiled`` ignores `_loader` for `file-path` and continues as normal. Otherwise, the first return value is yet another procedure. When this procedure is applied to no arguments, it should have the same effect as loading `file-path`. The second return value is either a symbol or `#f`; a symbol indicates that calling the returned procedure has the effect of declaring the module named by the symbol (which is potentially useful information to a load handler; see section 5.8).

While a `.zo`, `.so`, or `.dll` file is loaded (or while a thunk returned by `_loader` is invoked), the current ``load-relative`` directory is set to the directory of the original `file-path`.

`(load/cd``` `file-path``)` is the same as `(load `file-path`)`, but ``load/cd`` sets both the current directory and current ``load-relative`` directory to the directory of `file-path` before the file's expressions are evaluated.

`(read-eval-print-loop``)` starts a new `read`-`eval`-`print` loop using the current input, output, and error ports. When ``read-eval-print-loop`` starts, it installs a new error escape procedure (see section 6.7) that does not exit the `read`-`eval`-`print` loop. The ``read-eval-print-loop`` procedure does not return until ``eof`` is read as an input expression; then it returns void.

The `read-eval-print-loop` procedure is parameterized by the current prompt read handler, the current evaluation handler, and the current print handler; a custom `read`-`eval`-`print` loop can be implemented as in the following example (see also section 7.7.1):

      (parameterize ([current-prompt-read my-read]
                     [current-eval my-eval]
                     [current-print my-print])
        (read-eval-print-loop))

## 14.2 Exiting

`(exit``` [`v`]`)` passes `v` on to the current exit handler (see ``exit-handler`` in section 7.7.1.9). The default value for `v` is `#t`. If the exit handler does not escape or terminate the thread, void is returned.

The default exit handler quits MzScheme (or MrEd), using its argument as the exit code if it is between 1 and 255 inclusive (meaning ``failure''), or 0 (meaning ``success'') otherwise.

When MzScheme is embedded within another application, the default exit handler may behave differently.

## 14.3 Input Parsing

MzScheme's input parser obeys the following non-standard rules:

- Square brackets (``['' and ``]'') and curly braces (``{'' and ``}'') can be used in place of parentheses. An open square bracket must be closed by a closing square bracket and an open curly brace must be closed by a closing curly brace. Whether square brackets are treated as parentheses is controlled by the `read-square-bracket-as-paren` parameter (see section 7.7.1.3). Similarly, the parsing of curly braces is controlled with the `read-curly-brace-as-paren` parameter. By default, square brackets and curly braces are treated as parentheses.

- Vector constants can be unquoted, and a vector size can be specified with a decimal integer between the `#` and opening parenthesis. If the specified size is larger than the number of vector elements that are provided, the last specified element is used to fill the remaining vector slots. For example, `#4(1 2)` is equivalent to `#(1 2 2 2)`. If no vector elements are specified, the vector is filled with `0`. If a vector size is provided and it is smaller than the number of elements provided, the `exn:read` exception is raised.

- Boxed constants can be created using `#&`. The S-expression following `#&` is treated as a quoted constant and put into the new box. (Spaces following the `#&` are ignored.) Box reading is controlled with the `read-accept-box` boolean parameter (see section 7.7.1.3). Box reading is enabled by default. When box reading is disabled and `#&` is provided as input, the `exn:read` exception is raised.

- Expressions beginning with `#'` are wrapped with `syntax`, in the same way that expressions starting with `'` are wrapped with `quote`. Similarly, `#`` generates `quasisyntax`, `#,` generates `unsyntax`, and `#,@` generates `unsyntax-splicing`. See also section 12.2.1.2.

- The following character constants are recognized:

  - `#\nul` or `#\null` (ASCII 0)

  - `#\backspace` (ASCII 8)

  - `#\tab` (ASCII 9)

  - `#\newline` or `#\linefeed` (ASCII 10)

  - `#\vtab` (ASCII 11)

  - `#\page` (ASCII 12)

  - `#\return` (ASCII 13)

  - `#\space` (ASCII 32)

  - `#\rubout` (ASCII 127)

  Whenever `#\` is followed by at least two alphabetic characters, characters are read from the input port until the next non-alphabetic character is returned. If the resulting string of letters does not match one of the above constants (case-insensitively), the `exn:read` exception is raised.

  Character constants can also be specified through direct ASCII values in octal notation: `#`n1``n2``n3`` where `n1` is in the range [`0`, `3`] and `n2` and `n3` are in the range [`0`, `7`]. Whenever `#\` is followed by at least two characters in the range [`0`, `7`], the next character must also be in this range and the resulting octal number must be in the range 0008 to 3778.

- Within string constants, the following escape sequences are recognized in addition to ``\`"` and ``\``\``:

  - ``\`a`: alarm (ASCII 7)

  - ``\`b`: backspace (ASCII 8)

  - ``\`t`: tab (ASCII 9)

  - ``\`n`: linefeed (ASCII 10)

  - ``\`v`: vertical tab (ASCII 11)

  - ``\`f`: formfeed (ASCII 12)

  - ``\`r`: return (ASCII 13)

  - ``\`e`: escape (ASCII 27)

  - ``\``o``, ``\``oo``, or ``\``ooo``: ASCII for octal `o`, `oo`, or `ooo`, where each `o` is `0`, `1`, `2`, `3`, `4`, `5`, `6`, or `7`. The ``\``ooo`` form takes precedence over the ``\``oo`` form, and ``\``oo`` takes precedence over ``\``o``.

  - ``\`x`h`` or ``\`x`hh``: ASCII for hexadecimal `h` or `hh`, where each `h` is `0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`, `a`, `A`, `b`, `B`, `c`, `C`, `d`, `D`, `e`, `E`, `f`, or `F`. The ``\`x`hh`` form takes precedence over the ``\`x`h`` form.

  Furthermore, a backslash followed by a linefeed, carriage return or return-linefeed combination is elided, allowing string constants to span lines. Any other use of backslash within a string constant is an error.

- Numbers containing a decimal point or exponent (e.g., `1.3`, `2e78`) are normally read as inexact. If the `read-decimal-as-inexact` parameter is set to `#f`, then such numbers are instead read as exact. The parameter does not affect the parsing of numbers with an explicit exactness tag (`#e` or `#i`).

- A parenthesized sequence containing two delimited dots (```.`'') triggers infix parsing. A single `datum` must appear between the dots, and one or more `datum`s must appear before the first dot and after the last dot:

      (left-datum ···1 . first-datum . right-datum ···1)

  The resulting list consists of the `datum` between the dots, followed by the remaining `datum`s in order:

      (first-datum left-datum ···1 right-datum ···1)

  Consequently, the input expression `(1 . < . 2)` produces `#t`, and `(1 2 . + . 3 4 5)` produces `15`.

- When the `read-accept-dot` parameter is set to `#f`, then a delimited dot (```.`'') is disallowed in input. When the `read-accept-quasiquote` parameter is set to `#f`, then a backquote or comma is disallowed in input. These modes simplify Scheme's input model for students.

- MzScheme's identifier and symbol syntax is considerably more liberal than the syntax specified by R5RS. When input is scanned for tokens, the following characters delimit an identifier:

  ``

  " , ' ` ( ) [ ] { } space tab return newline page vtab

  In addition, an identifier cannot start with a hash mark (```#`'') unless the hash mark is immediately followed by a percent sign (```%`''). The only other special characters are backslash (```\`'') or quoting vertical bars (``|''); any other character is used as part of an identifier.

  Symbols containing special characters (including delimiters) are expressed using an escaping backslash (```\`'') or quoting vertical bars (``|''):

  - A backslash preceding any character includes that character in the symbol literally; double backslashes produce a single backslash in the symbol.

  - Characters between a pair of vertical bars are included in the symbol literally. Quoting bars can be used for any part of a symbol, or the whole symbol can be quoted. Backslashes and quoting bars can be mixed within a symbol, but a backslash is not a special character within a pair of quoting bars.

  Characters quoted with a backslash or a vertical bar always preserve their case, even when identifiers are read case-insensitively (the default).

  An input token constructed in this way is an identifier when it is not a numerical constant (following the extended number syntax described in section 3.3). A token containing a backslash or vertical bars is never treated as a numerical constant.

  Examples:

  - `(`quote` a`\`(b)` produces the same symbol as `(`string->symbol` "a(b")`.

  - `(`quote` A`\`B)` produces the same symbol as `(`string->symbol` "aB")` when identifiers are read without case-sensitivity.

  - `(`quote` a`\` b)`, `(`quote` |a b|)`, and `(`quote` a| |b)` all produce the same symbol as `(`string->symbol` "a b")`.

  - `(`quote` |a||b|)` is the same as `(quote ab)`.

  - `(`quote` 10)` is the number 10, but `(`quote` |10|)` produces the same symbol as `(`string->symbol` "10")`.

  Whether a vertical bar is used as a special or normal symbol character is controlled with the `read-accept-bar-quote` boolean parameter (see section 7.7.1.3). Vertical bar quotes are enabled by default. Quoting backslashes cannot be disabled.

- By default, symbols are read case-insensitively (i.e., uppercase characters are downcased). Case sensitivity for reading can be controlled in three ways:

  - Quoting part of a symbol with an escaping backslash (```\`'') or quoting vertical bar (``|'') always preserves the case of the quoted portion, as described above.

  - The sequence `#cs` can be used as a prefix for any expression to make reading symbols within the expression case-sensitive. A `#ci` prefix similarly makes reading symbols in an expression case-insensitive. Whitespace can appear between a `#cs` or `#ci` prefix and its expression, and prefixes can be nested. Backslash and vertical-bar quotes override a `#ci` prefix.

  - When the `read-case-sensitive` parameter (see section 7.7.1.3) is set to `#t`, then case is preserved when reading symbols. The default is `#f`, and it is set to `#f` while loading a module (see section 5.8). A `#cs` or `#ci` prefix overrides the parameter setting, as does backslash or vertical-bar quoting.

  Case conversions are not sensitive to the current locale (see section 7.7.1.11).

- Expressions of the form `#rx`string`` are literal regexp values (see section 10) where `string` is a string constant. The regexp produced by `#rx`string`` is the same as produced by `(regexp `string`)`. If `string` is not a valid pattern, the `exn:read` exception is raised.

- Expressions of the form `#hash((`key-datum` . `val-datum`) `···`)` are literal immutable hash tables. The hash table maps each `key-datum` to its `val-datum`, comparing keys with ``equal?``. The table is constructed by adding each `key-datum` mapping from left to right, so later mappings can hide earlier mappings if the `key-datum`s are ``equal?``. An expression of the form `#hasheq((`key-datum` . `val-datum`) `···`)` produces an immutable hash table with keys compared using ``eq?``. If the value of `read-square-bracket-as-paren` parameter (see section 7.7.1.3) is true, matching parenthesese in a `#hash` or `#hasheq` constant can be replaced by matching square brackets. Similarly, matching curly braces can be used if `read-curly-brace-as-paren` is true.

- S-expressions with shared structure are expressed using `#`n`=` and `#`n`#`, where `n` is a decimal integer. See section 14.5.

- Expressions of the form `#%`x`` are symbols, where `x` can be a symbol or a number.

- Expressions beginning with `#~` are interpreted as compiled MzScheme code. See section 14.6.

- Multi-line comments are started with `#`|`` and terminated with ``|`#`. Comments of this form can be nested arbitrarily.

- If the first line of a ``load``ed file begins with `#!`, it is ignored by the default load handler. If an ignored line ends with a backslash (```\`''), then the next line is also ignored. (The `#!` convention is for shell scripts; see Chapter 18 for details.)

Reading from a custom port can produce arbitrary values generated by the port; see section 11.1.6 for details. If the port generates a non-character value in a position where a character is required (e.g., within a string), the `exn:read:non-char` exception is raised.

## 14.4 Output Printing

MzScheme's printer obeys the following non-standard rules:

- A vector can be printed by ``write`` and ``print`` using the shorthand described in section 14.3, where the vector's length is printed between the leading `#` and the opening parenthesis and repeated tail elements are omitted. For example, `#(1 2 2 2)` is printed as `#4(1 2)`. The ``display`` procedure does not output vectors using this shorthand. Shorthand vector printing is controlled with the `print-vector-length` boolean parameter (see section 7.7.1.4). Shorthand vector printing is enabled by default.

- Boxes (see section 3.9) can be printed with the `#&` notation (see section 14.3). When box printing is disabled, all boxes are printed as `#<box>`. Box printing is controlled with the `print-box` boolean parameter (see section 7.7.1.4). Box printing is enabled by default.

- Structures (see Chapter 4) can be printed using vector notation. In the vector, the first item is a symbol of the form `struct:`s`` -- where `s` is the name of the structure -- and the remaining elements are the elements of the structure, but the vector exposes only as much information about the structure as the current inspector can access (see section 4.5). When structure printing is disabled, or when no part of the structure is accessible to the current inspector, a structure is printed as `#<struct:`s`>`. Structure printing is controlled with the `print-struct` boolean parameter (see section 7.7.1.4). Structure printing is disabled by default.

- Symbols containing spaces or special characters ``write`` using escaping backslashes and quoting vertical bars. When the `read-case-sensitive` parameter is set to `#f`, then symbols containing uppercase characters also use escaping backslashes or quoting vertical bars. In addition, symbols are quoted with vertical bars or a leading backslash when they would otherwise print the same as a numerical constant. If the value of the `read-accept-bar-quote` boolean parameter is `#f` (see section 7.7.1.3), then backslashes are always used to escape special characters instead of quoting them with vertical bars, and a vertical bar is not treated as a special character. See section 14.3 for more information about symbol parsing. Symbols ``display`` without escaping or quoting special characters.

- Characters with the special names described in section 14.3 ``write`` using the same name. (Some characters have multiple names; the `#\newline` and `#\nul` names are used instead of `#\linefeed` and `#\null`). Other ``printable'' characters ``write`` as `#\` followed by the single-byte character value, and ``unprintable'' characters are written in octal notation; unprintable-character detection depends on locale sensitivity (see section 7.7.1.11), and when the current locale is disabled, a character whose ``char->integer`` value is greater than 127 is always treated as unprintable. All characters ``display`` as the single-byte character value.

- Strings containing ``unprintable'' characters (see above) ``write`` using the escape sequences described in section 14.3. All strings ``display`` as their literal character sequences.

- Regular expressions print using the form `#rx`string``, where `string` is the ``write`` form of the regexp's source string.

- When the `print-hash-table` parameter is set to true (see section 7.7.1.4), hash tables print using the form `#hash((`key` . `val`) `···`)` or `#hasheq((`key` . `val`) `···`)` for tables using ``equal?`` or ``eq?`` key comparisions, respectively. If the `print-hash-table` parameter's value is `#f`, hash tables print as `#<hash-table>`. Hash tables with weakly held keys always print as `#<hash-table>`.

- S-expressions with shared structure can be printed using `#`n`=` and `#`n`#`, where `n` is a decimal integer. See section 14.5.

## 14.5 Data Sharing in Input and Output

MzScheme can read and print graphs, S-expressions with shared structure (e.g., a cycle). Graphs are described by tagging the shared structure once with `#`n`=` (using some decimal integer `n` with no more than eight digits) and then referencing it later with `#`n`#` (using the same number `n`). For example, the following S-expression describes the infinite list of ones:

    #0=(1 . #0#)

If this graph is entered into MzScheme's `read`-`eval`-`print` loop, MzScheme's compiler will loop forever, trying to compile an infinite expression. In contrast, the following expression defines `ones` to the infinite list of ones, using `quote` to hide the infinite list from the compiler:

    (define ones (quote #0=(1 . #0#)))

A tagged structure can be referenced multiple times. Here, `v` is defined to be a vector containing the same ``cons`` cell in all three slots:

    (define v #(#1=(cons 1 2) #1# #1#))

A tag `#`n`=` must appear to the left of all references `#`n`#`, and all references must appear in the same top-level S-expression as the tag. By default, MzScheme's printer will display a value without showing the shared structure:

    #((1 . 2) (1 . 2) (1 . 2))

Graph reading and printing are controlled with the `read-accept-graph` and `print-graph` boolean parameters (see section 7.7.1.4). Graph reading is enabled by default, and graph printing is disabled by default. However, when the printer encounters an graph containing a cycle, graph printing is automatically enabled, temporarily. (For this reason, the `display`, `write`, and `print` procedures require memory proportional to the depth of the value being printed.) When graph reading is disabled and a graph is provided as input, the `exn:read` exception is raised.

If the `n` in a `#`n`=` form or a `#`n`#` form contains more than eight digits, the `exn:read` exception is raised. If a `#`n`#` form is not preceded by a `#`n`=` form using the same `n`, the `exn:read` exception is raised. If two `#`n`=` forms are in the same expression for the same `n`, the `exn:read` exception is raised.

## 14.6 Compilation

Normally, compilation happens automatically: when an S-expression is evaluated, it is first compiled and then the compiled code is executed. However, MzScheme can also write and read compiled code. MzScheme can read compiled code much faster than reading S-expression code and compiling it, so compilation can be used to speed up program loading. The MzLib procedure ``compile-file`` (see Chapter mz:compilefile in PLT MzLib: Libraries Manual) is sufficient for most compilation purposes.

- `(compile``` `expr``)` compiles `expr`, where `expr` is any S-expression that can be passed to ``eval``. The result is a compiled expression Scheme value. This value can be passed to ``eval`` to evaluate the compiled expression.

- `(compiled-expression?``` `v``)` returns `#t` if `v` is a compiled expression, `#f` otherwise.

When a compiled expression is written to an output port, the written form starts with `#~`. These expressions are essentially assembly code for the MzScheme interpreter, and reading such an expression produces a compiled expression. When a compiled expression contains syntax object constants, the `#~` form of the expression drops location information and properties for the syntax objects (see section 12.2 and section 12.6.2).

Never ask MzScheme to evaluate an expression starting with `#~` unless ``compile`` generated the expression. To keep users from accidentally specifying bad instructions, ``read`` will not accept expressions beginning with `#~` unless it is specifically enabled through the `read-accept-compiled` boolean parameter (see section 7.7.1.3). When the default load handler is used to load a file, compiled expression reading is automatically (temporarily) enabled as each expression is read.

A compiled code object may contain uninterned symbols (see section 3.6) that were created by ``gensym`` or ``string->uninterned-symbol``. When the compiled object is read via `#~`, each uninterned symbol in the original expression is mapped to a new uninterned symbol, where multiple instances of a single symbol are consistently mapped to the same new symbol. The original and new symbols have the same printed representation.

Due to the above restrictions, do not use ``gensym`` or ``string->uninterned-symbol`` to construct an indentifier for a top-level or module binding. Instead, generate distinct identifiers either with `generate-temporaries` (see section 12.2.2) or by applying the result of a `make-syntax-introducer` (see section 12.6) to an existing identifier.

## 14.7 Dynamic Extensions

A dynamically-linked extension library is loaded into MzScheme with `(load-extension``` `file-path``)`. The separate document Inside PLT MzScheme contains information about writing MzScheme extensions. An extension can only be loaded once during a MzScheme session, although the extension-writer can provide functionality to handle extra calls to ``load-extension`` for a single extension.

As with ``load``, the current ``load-relative`` directory (the value of the `current-load-relative-directory` parameter; see section 7.7.1.6) is set while the extension is loaded. The `load-relative-extension` procedure is like ``load-extension``, but it loads an extension with a pathname that is relative to the current ``load-relative`` directory instead of the current directory.

The ``load-extension`` procedure actually just dispatches to the current load extension handler (see section 7.7.1.6). The result of calling ``load-extension`` is determined by the extension. If the extension cannot be loaded, the `exn:i/o:filesystem` exception is raised. The `detail` field of the exception is `'wrong-version` if the load fails because the extension has the wrong version.

## 14.8 Saving and Restoring Program Images

An image is a memory dump from a running MzScheme program that can be later restored (one or more times) to continue running the program from the point of the dump. Images are only supported for statically-linked Unix versions of MzScheme (and MrEd). There are a few special restrictions on images:

- All files, TCP connections, and UDP connections must be closed when an image is created.

- No dynamic extensions can be loaded before an image is created.

- No operating system subprocesses can be active when an image is created.

`(write-image-to-file``` `file-path``` [`cont-proc`]`)` copies the state of the entire MzScheme process 37 to `file-path`, replacing `file-path` if it already exists. If images are not supported, the `exn:misc:unsupported` exception is raised. If `cont-proc` is `#f`, then the MzScheme or MrEd process exits immediately after creating the image. Otherwise, `cont-proc` must be a procedure of no arguments, and the return value(s) of the call to ``write-image-to-file`` is `(cont-proc)`. The default value for `cont-proc` is ``void``.

`(read-image-from-file``` `file-path arg-vector``)` restores the image saved to `file-path`. Once the image is restored, execution of the original program continues with the return from ``write-image-to-file``; the return value in the restored program is the a vector of strings `arg-vector`. A successful call to ``read-image-from-file`` never returns because the restored program is overlayed over the current program. The vector `arg-vector` must contain no more than 20 strings, and the total length of the strings must be no more than 2048 characters.

If an error is encountered while reading or writing an image, the `exn:i/o:filesystem` exception is raised or `exn:misc` exception is raised. Certain errors during ``read-image-from-file`` are unrecoverable; in case of such errors, MzScheme prints an error message and exits immediately.

An image can also be restored by starting the stand-alone version of MzScheme or MrEd with the `--restore` flag followed by the image filename. The return value from ``write-image-to-file`` in the restored program is a vector of strings that are the extra arguments provided on the command line after the image filename (if any).

34 The ``eval`` procedure actually calls the current evaluation handler (see section 7.7.1.5) with `e` to evaluate the expression.

35 The ``load`` procedure actually just sets the current ``load-relative`` directory and calls the current load handler (see section 7.7.1.6) with `file-path` to load the file. The description of ``load`` here is actually a description of the default load handler.

36 The ``load/use-compiled`` procedure actually just calls the current load/use-compiled handler (see section 7.7.1.6). The default handler, in turn, calls the load or load-extension handler, depending on the type of file that is loaded.

37 The set of environment variables is not saved. When an image is restored, the environment variables of the restoring program are transferred into the restored program.