13.2 Evaluation and Compilation

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▼ | 13 | Reflection and Security

13.1 | Namespaces
13.2 | Evaluation and Compilation
13.3 | The racket/ load Language
13.4 | Module Names and Loading
13.5 | Impersonators and Chaperones
13.6 | Security Guards
13.7 | Custodians
13.8 | Thread Groups
13.9 | Structure Inspectors
13.10 | Code Inspectors
13.11 | Sandboxed Evaluation

On this page:

current-eval
eval
eval-syntax
current-load
load
load-relative
load/ cd
current-load-extension
load-extension
load-relative-extension
current-load/ use-compiled
load/ use-compiled
current-load-relative-directory
use-compiled-file-paths
current-compiled-file-roots
read-eval-print-loop
current-prompt-read
current-get-interaction-input-port
current-read-interaction
current-print
current-compile
compile
compile-syntax
compiled-expression?
compile-enforce-module-constants
compile-allow-set!-undefined
compile-context-preservation-enabled
eval-jit-enabled
load-on-demand-enabled

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#### 13.2`` Evaluation and Compilation

> parameter (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.

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

> > > See also Namespaces in The Racket Guide.

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:

- 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.

> procedure (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.

> parameter (current-load)
> | → |  |
> (path? (or/c #f
> symbol?
> (cons/c (or/c #f symbol?)
> (non-empty-listof symbol?)))
> . -> .
> any)
> (current-load proc) → void?
> | proc |  | : |  |
> (path? (or/c #f
> symbol?
> (cons/c (or/c #f symbol?)
> (non-empty-listof symbol?)))
> . -> .
> 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 or a list 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.

When loading a module from a stream that starts with a compiled module that contains submodules, the load handler should load only the requested module, where a symbol as the load handler’s indicates the root module and a list indicates a submodule whose path relative to the root module is given by the cdr of the list. The list starts with #f when a submodule should be loaded only if it can be loaded independently (i.e., from compiled form—never from source); if the submodule cannot be loaded independently, the load handler should return without loading from a file. When the expected module name is a list that starts with a symbol, the root module and any other submodules can be loaded from the given file, which might be from source, and the load handler still should not complain if the expected submodule is not found.

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 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)
  > (read-accept-lang #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.

> procedure (load file) → any
> file : path-string?

> > > See also Namespaces in The Racket Guide.

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.

> procedure (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.

> procedure (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.

> parameter (current-load-extension)
> → (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.

> procedure (load-extension file) → any
> file : path-string?

Sets current-load-relative-directory like load, and calls the extension-load handler in tail position.

> procedure (load-relative-extension file) → any
> file : path-string?

Like load-extension, but resolves file using current-load-relative-directory like load-relative.

> parameter (current-load/use-compiled)
> | → |  |
> (path? (or/c #f
> symbol?
> (cons/c (or/c #f symbol?)
> (non-empty-listof symbol?)))
> . -> . any)
> (current-load/use-compiled proc) → void?
> | proc |  | : |  |
> (path? (or/c #f
> symbol?
> (cons/c (or/c #f symbol?)
> (non-empty-listof symbol?)))
> . -> . 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 current-compiled-file-roots and use-compiled-file-paths parameters relative to file, where the former supplies “roots” for compiled files and the latter provides subdirectories. A “root” can be an absolute path, in which case file’s directory is combined with reroot-path and the root as the second argument; if the “root” is a relative path, then the relative path is instead suffixed onto the directory of file. The roots are tried in order, and the subdirectories are checked in order within each root. 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 the default compiled-load handler loads a module from a bytecode (i.e., ".zo") file, the handler records the bytecode file path in the current namespace’s module registry. More specifically, the handler records the path for the top-level module of the loaded module, which is an enclosing module if the loaded module is a submodule. Thereafter, loads via the default compiled-load handler for modules within the same top-level module use the recorded file, independent of the file that otherwise would be selected by the compiled-load handler (e.g., even if the use-compiled-file-paths parameter value changes). The default module name resolver transfers bytecode-file information when a module declaration is attached to a new namespace. This protocol supports independent but consistent loading of submodules from bytecode files.

> procedure (load/use-compiled file) → any
> file : path-string?

Calls the current compiled-load handler in tail position.

> parameter (current-load-relative-directory)
> → (or/c (and/c path-string? complete-path?) #f)
> (current-load-relative-directory path) → void?
> path : (or/c (and/c path-string? complete-path?) #f)

A parameter that is set by load, load-relative, load-extension, load-relative-extension, and the default compiled-load handler, and used by load-relative, load-relative-extension, and the default compiled-load handler.

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.)

> parameter (use-compiled-file-paths) → (listof path?)
> (use-compiled-file-paths paths) → void?
> paths : (listof path-string?)

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).

> parameter (current-compiled-file-roots) → (listof (or/c path? 'path))
> (current-compiled-file-roots paths) → void?
> paths : (listof (or/c path-string? 'same))

A list of paths and 'sames that is is used by the default compiled-load handler (see current-load/use-compiled).

The parameter is normally initialized to (list 'same), but the parameter’s initial value can be adjusted by the PLTCOMPILEDROOTS environment variable or the --compiled or -R command-line flag for racket. If the environment variable is defined and not overridden by a command-line flag, it is parsed by first replacing any @(version) with the result of (version), then using path-list-string->path-list with a default path list (list (build-path 'same)) to arrive at the parameter’s initial value.

> procedure (read-eval-print-loop) → any

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.

> parameter (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))

If the input and output ports are both terminals (in the sense of terminal-port?) and if the output port appears to be counting lines (because port-next-location returns a non-#f line and column), then the output port’s line is incremented and its column is reset to 0 via set-port-next-location! before returning the read result.

> parameter (current-get-interaction-input-port) → (-> input-port?)
> (current-get-interaction-input-port proc) → void?
> proc : (-> input-port?)

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.

> parameter (current-read-interaction) → (any/c input-port? -> any)
> (current-read-interaction proc) → void?
> proc : (any/c input-port? -> any)

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))

> parameter (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>.

> parameter (current-compile)
> → (any/c boolean? . -> . compiled-expression?)
> (current-compile proc) → void?
> proc : (any/c boolean? . -> . compiled-expression?)

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 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.

For internal testing purposes, when the PLT_VALIDATE_COMPILE environment variable is set, the default compilation handler runs a bytecode validator on its own compilation results.

> procedure (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.

> procedure (compile-syntax stx) → compiled-expression?
> stx : syntax?

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

> procedure (compiled-expression? v) → boolean?
> v : any/c

Returns #t if v is a compiled form, #f otherwise.

> parameter (compile-enforce-module-constants) → boolean?
> (compile-enforce-module-constants on?) → void?
> on? : any/c

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.

> parameter (compile-allow-set!-undefined) → boolean?
> (compile-allow-set!-undefined allow?) → void?
> allow? : any/c

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.

> parameter (compile-context-preservation-enabled) → boolean?
> (compile-context-preservation-enabled on?) → void?
> on? : any/c

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.

> parameter (eval-jit-enabled) → boolean?
> (eval-jit-enabled on?) → void?
> on? : any/c

> > > See also The Bytecode and Just-in-Time (JIT) Compilers in The Racket Guide.

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. A true parameter value is effective only on platforms for which the JIT is supported, and changing the value from its initial setting affects only forms that are outside of module.

The default is #t, unless the JIT is not supported by the current platform, unless it is disabled through the -j/--no-jit command-line flag to stand-alone Racket (or GRacket), and unless it is disabled through the PLTNOMZJIT environment variable (set to any value).

> parameter (load-on-demand-enabled) → boolean?
> (load-on-demand-enabled on?) → void?
> on? : any/c

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.
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