13.2 Evaluation and Compilation

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.

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:

• If top-level-form is a pair whose car is a symbol or identifier, and if applying namespace-syntax-introduce to the (datum->syntax-converted) identifier produces an identifier bound to module in a phase level that corresponds to namespace’s base phase, then only that identifier is enriched.

• For any other top-level-form, namespace-syntax-introduce is applied to the entire syntax object.

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.

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 both read-accept-compiled and read-accept-reader 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:

• The read-syntax from the file is additionally parameterized as follows (to provide consistent reading of module source):

  (current-readtable #f)

  (read-case-sensitive #t)

  (read-square-bracket-as-paren #t)

  (read-curly-brace-as-paren #t)

  (read-accept-box #t)

  (read-accept-compiled #t)

  (read-accept-bar-quote #t)

  (read-accept-graph #t)

  (read-decimal-as-inexact #t)

  (read-accept-dot #t)

  (read-accept-infix-dot #t)

  (read-accept-quasiquote #t)

  (read-accept-reader #t)

• If the read result is not a module form, or if a second read-syntax does not produce an end-of-file, then the exn:fail exception is raised without evaluating the form that was read from the file. (In previous versions, the module declaration was checked to match the name given as the second argument to the load handler, but this check is no longer performed.)

• The lexical information of the initial module identifier is enriched with a binding for module, so that the form corresponds to a module declaration independent of the current namespace’s bindings.

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.

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.

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.

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.)

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

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

(let ([in (current-input-port)])

((current-read-interaction) (object-name in) in))

The default read interaction handler accepts src and in and returns

(parameterize ([read-accept-reader #t])

(read-syntax src in))

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>.

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 #~. These forms are essentially assembly code for Racket, and reading such an form produces a compiled form (as long as the read-accept-compiled parameter is set to #t).

When a compiled form contains syntax object constants, the #~-marshaled form drops source-location information and properties (Syntax Object Properties) for the syntax objects.

Compiled code parsed from #~ may contain references to unexported or protected bindings from a module. At read time, such references are associated with the current code inspector (see current-code-inspector), and the code will only execute if that inspector controls the relevant module invocation (see Code Inspectors).

A compiled-form object may contain uninterned symbols (see Symbols) that were created by gensym or string->uninterned-symbol. When the compiled object is read via #~, each uninterned symbol in the original form 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. Unreadable symbols, which are typically generated indirectly during expansion and compilation, are saved and restored consistently through #~.

Due to the restrictions on uninterned symbols in #~, do not use gensym or string->uninterned-symbol to construct an identifier for a top-level or module binding. Instead, generate distinct identifiers either with generate-temporaries or by applying the result of make-syntax-introducer to an existing identifier; those functions will lead to top-level and module bindings with unreadable symbolic names.

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.