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13.2 Evaluation and Compilation

(current-eval)  (any/c . -> . any)
(current-eval proc)  void?
  proc : (any/c . -> . any)
A parameter that determines the current evaluation handler. The evaluation handler is a procedure that takes a top-level form and evaluates it, returning the resulting values. The evaluation handler is called by eval, eval-syntax, the default load handler, and read-eval-print-loop to evaluate a top-level form. The handler should evaluate its argument in tail position.

The top-level-form provided to the handler can be a syntax object, a compiled form, a compiled form wrapped as a syntax object, or an arbitrary datum.

The default handler converts an arbitrary datum to a syntax object using datum->syntax, and then enriches its lexical information in the same way as eval. (If top-level-form is a syntax object, then its lexical information is not enriched.) The default evaluation handler partially expands the form to splice the body of top-level begin forms into the top level (see expand-to-top-form), and then individually compiles and evaluates each spliced form before continuing to expand, compile, and evaluate later forms.

(eval top-level-form [namespace])  any
  top-level-form : any/c
  namespace : namespace? = (current-namespace)

Calls the current evaluation handler to evaluate top-level-form. The evaluation handler is called in tail position with respect to the eval call, and parameterized to set current-namespace to namespace.

If top-level-form is a syntax object whose datum is not a compiled form, then its lexical information is enriched before it is sent to the evaluation handler:

For interactive evaluation in the style of read-eval-print-loop and load, wrap each expression with #%top-interaction, which is normally bound to #%top-interaction, before passing it to eval.

(eval-syntax stx [namespace])  any
  stx : syntax?
  namespace : namespace? = (current-namespace)
Like eval, except that stx must be a syntax object, and its lexical context is not enriched before it is passed to the evaluation handler.

(current-load)  (path? (or/c symbol? #f) . -> . any)
(current-load proc)  void?
  proc : (path? (or/c symbol? #f) . -> . any)
A parameter that determines the current load handler to load top-level forms from a file. The load handler is called by load, load-relative, load/cd, and the default compiled-load handler.

A load handler takes two arguments: a path (see Paths) and an expected module name. The expected module name is a symbol when the call is to load a module declaration in response to a require (in which case the file should contain a module declaration), or #f for any other load.

The default load handler reads forms from the file in read-syntax mode with line-counting enabled for the file port, unless the path has a ".zo" suffix. It also parameterizes each read to set read-accept-compiled, read-accept-reader, and read-accept-lang to #t. In addition, if load-on-demand-enabled is #t, then read-on-demand-source is effectively set to the cleansed, absolute form of path during the read-syntax call. After reading a single form, the form is passed to the current evaluation handler, wrapping the evaluation in a continuation prompt (see call-with-continuation-prompt) for the default continuation prompt tag with handler that propagates the abort to the continuation of the load call.

If the second argument to the load handler is a symbol, then:

If the second argument to the load handler is #f, then each expression read from the file is wrapped with #%top-interaction, which is normally bound to #%top-interaction, before passing it to the evaluation handler.

The return value from the default load handler is the value of the last form from the loaded file, or #<void> if the file contains no forms. If the given path is a relative path, then it is resolved using the value of current-directory.

(load file)  any
  file : path-string?

Calls the current load handler in tail position. The call is parameterized to set current-load-relative-directory to the directory of file, which is resolved relative to the value of current-directory.

(load-relative file)  any
  file : path-string?
Like load/use-compiled, but when file is a relative path, it is resolved using the value of current-load-relative-directory instead of the value of current-directory if the former is not #f, otherwise current-directory is used.

(load/cd file)  any
  file : path-string?
Like load, but load/cd sets both current-directory and current-load-relative-directory before calling the load handler.

  (path? (or/c symbol? #f) . -> . any)
(current-load-extension proc)  void?
  proc : (path? (or/c symbol? #f) . -> . any)
A parameter that determines a extension-load handler, which is called by load-extension and the default compiled-load handler.

An extension-load handler takes the same arguments as a load handler, but the file should be a platform-specific dynamic extension, typically with the file suffix ".so" (Unix), ".dll" (Windows), or ".dylib" (Mac OS X). The file is loaded using internal, OS-specific primitives. See Inside: Racket C API for more information on dynamic extensions.

(load-extension file)  any
  file : path-string?
Sets current-load-relative-directory like load, and calls the extension-load handler in tail position.

(load-relative-extension file)  any
  file : path-string?
Like load-exension, but resolves file using current-load-relative-directory like load-relative.

  (path? (or/c symbol? #f) . -> . any)
(current-load/use-compiled proc)  void?
  proc : (path? (or/c symbol? #f) . -> . any)
A parameter that determines the current compiled-load handler to load from a file that may have a compiled form. The compiled-load handler is called by load/use-compiled.

The protocol for a compiled-load handler is the same as for the load handler (see current-load), except that a compiled-load handler is expected to set current-load-relative-directory itself. The default compiled-load handler, however, checks for a ".ss" file when the given path ends with ".rkt", no ".rkt" file exists, and when the handler’s second argument is a symbol. In addition, the default compiled-load handler checks for ".zo" (bytecode) files and ".so" (native Unix), ".dll" (native Windows), or ".dylib" (native Mac OS X) files.

The check for a compiled file occurs whenever the given path file ends with any extension (e.g., ".rkt" or ".scrbl"), and the check consults the subdirectories indicated by the use-compiled-file-paths parameter relative to file. The subdirectories are checked in order. A ".zo" version of the file (whose name is formed by passing file and #".zo" to path-add-suffix) is loaded if it exists directly in one of the indicated subdirectories, or a ".so"/".dll"/".dylib" version of the file is loaded if it exists within a "native" subdirectory of a use-compiled-file-paths directory, in an even 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. If both ".zo" and ".so"/".dll"/".dylib" files are available, the ".so"/".dll"/".dylib" file is used. If file ends with ".rkt", no such file exists, the handler’s second argument is a symbol, and a ".ss" file exists, then ".zo" and ".so"/".dll"/".dylib" files are used only with names based on file with its suffixed replaced by ".ss".

While a ".zo", ".so", ".dll", or ".dylib" file is loaded, the current load-relative directory is set to the directory of the original file. If the file to be loaded has the suffix ".ss" while the requested file has the suffix ".rkt", then the current-module-declare-source parameter is set to the full path of the loaded file, otherwise the current-module-declare-source parameter is set to #f.

If the original file is loaded or a ".zo" variant is loaded, the load handler is called to load the file. If any other kind of file is loaded, the extension-load handler is called.

(load/use-compiled file)  any
  file : path-string?
Calls the current compiled-load handler in tail position.

When a new path or string is provided as the parameter’s value, it is immediately expanded (see Paths) and converted to a path. (The directory need not exist.)

A list of relative paths, which defaults to (list (string->path "compiled")). It is used by the compiled-load handler (see current-load/use-compiled).

Starts a new REPL using the current input, output, and error ports. The REPL wraps each expression to evaluate with #%top-interaction, which is normally bound to #%top-interaction, and it wraps each evaluation with a continuation prompt using the default continuation prompt tag and prompt handler (see call-with-continuation-prompt). The REPL also wraps the read and print operations with a prompt for the default tag whose handler ignores abort arguments and continues the loop. The read-eval-print-loop procedure does not return until eof is read, at which point it returns #<void>.

The read-eval-print-loop procedure can be configured through the current-prompt-read, current-eval, and current-print parameters.

(current-prompt-read)  (-> any)
(current-prompt-read proc)  void?
  proc : (-> any)
A parameter that determines a prompt read handler, which is a procedure that takes no arguments, displays a prompt string, and returns a top-level form to evaluate. The prompt read handler is called by read-eval-print-loop, and after printing a prompt, the handler typically should call the read interaction handler (as determined by the current-read-interaction parameter) with the port produced by the interaction port handler (as determined by the current-get-interaction-input-port parameter).

The default prompt read handler prints >  and returns the result of

(let ([in ((current-get-interaction-input-port))])
  ((current-read-interaction) (object-name in) in))

A parameter that determines the interaction port handler, which returns a port to use for read-eval-print-loop inputs.

The default interaction port handler returns the current input port. In addition, if that port is the initial current input port, the initial current output and error ports are flushed.

The racket/gui/base library adjusts this parameter’s value by extending the current value. The extension wraps the result port so that GUI events can be handled when reading from the port blocks.

A parameter that determines the current read interaction handler, which is procedure that takes an arbitrary value and an input port and returns an expression read from the input port.

The default read interaction handler accepts src and in and returns

(parameterize ([read-accept-reader #t]
               [read-accept-lang #f])
  (read-syntax src in))

(current-print)  (any/c -> any)
(current-print proc)  void?
  proc : (any/c -> any)
A parameter that determines the print handler that is called by read-eval-print-loop to print the result of an evaluation (and the result is ignored).

The default print handler prints the value to the current output port (as determined by the current-output-port parameter) and then outputs a newline, except that it prints nothing when the value is #<void>.

A parameter that determines the current compilation handler. The compilation handler is a procedure that takes a top-level form and returns a compiled form; see see Compilation for more information on compilation.

The compilation handler is called by compile, and indirectly by the default evaluation handler and the default load handler.

The handler’s second argument is #t if the compiled form will be used only for immediate evaluation, or #f if the compiled form may be saved for later use; the default compilation handler is optimized for the special case of immediate evaluation.

When a compiled form is written to an output port, the written form starts with #~. See Printing Compiled Code for more information.

(compile top-level-form)  compiled-expression?
  top-level-form : any/c
Like eval, but calls the current compilation handler in tail position with top-level-form.

Like eval-syntax, but calls the current compilation handler in tail position with stx.

(compiled-expression? v)  boolean?
  v : any/c
Returns #t if v is a compiled form, #f otherwise.

A parameter that determines how a module declaration is compiled.

When constants are enforced, and when the macro-expanded body of a module contains no set! assignment to a particular variable defined within the module, then the variable is marked as constant when the definition is evaluated. Afterward, the variable’s value cannot be assigned or undefined through module->namespace, and it cannot be defined by redeclaring the module.

Enforcing constants allows the compiler to inline some variable values, and it allows the native-code just-in-time compiler to generate code that skips certain run-time checks.

A parameter that determines how a set! expression is compiled when it mutates a global variable. If the value of this parameter is a true value, set! expressions for global variables are compiled so that the global variable is set even if it was not previously defined. Otherwise, set! expressions for global variables are compiled to raise the exn:fail:contract:variable exception if the global variable is not defined at the time the set! is performed. Note that this parameter is used when an expression is compiled, not when it is evaluated.

A parameter that determines whether compilation should avoid function-call inlining and other optimizations that may cause information to be lost from stack traces (as reported by continuation-mark-set->context). The default is #f, which allows such optimizations.

(eval-jit-enabled)  boolean?
(eval-jit-enabled on?)  void?
  on? : any/c
A parameter that determines whether the native-code just-in-time compiler (JIT) is enabled for code (compiled or not) that is passed to the default evaluation handler. The default is #t, unless the JIT is disabled through the -j/--no-jit command-line flag to stand-alone Racket (or GRacket), or through the PLTNOMZJIT environment variable (set to any value).

A parameter that determines whether the default load handler sets read-on-demand-source. See current-load for more information. The default is #t, unless it is disabled through the -d/--no-delay command-line flag.