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1.8.1 Syntax Classes
expr/ c
1.8.2 Literal Sets

1.8 Library Syntax Classes and Literal Sets

1.8.1 Syntax Classes



Matches anything except a keyword literal (to distinguish expressions from the start of a keyword argument sequence). The term is not otherwise inspected, since it is not feasible to check if it is actually a valid expression.

Match syntax satisfying the corresponding predicates.



Alias for identifier.




(static predicate description)  (attributes value)

  predicate : (-> any/c any/c)
  description : (or/c string? #f)
The static syntax class matches an identifier that is bound in the syntactic environment to static information (see syntax-local-value) satisfying the given predicate. If the term does not match, the description argument is used to describe the expected syntax.

When used outside of the dynamic extent of a macro transformer (see syntax-transforming?), matching fails.

The attribute value contains the value the name is bound to.


(expr/c contract-expr    
  [#:positive pos-blame    
  #:negative neg-blame    
  #:name expr-name    
  #:macro macro-name    
  #:context ctx])  (attributes c)
  contract-expr : syntax?
  pos-blame : (or/c syntax? string? module-path-index? 'from-macro 'use-site 'unknown)
   = 'use-site
  neg-blame : (or/c syntax? string? module-path-index? 'from-macro 'use-site 'unknown)
   = 'from-macro
  expr-name : (or/c identifier? string? symbol?) = #f
  macro-name : (or/c identifier? string? symbol?) = #f
  ctx : (or/c syntax? #f) = determined automatically
Accepts an expression (expr) and computes an attribute c that represents the expression wrapped with the contract represented by contract-expr.

The contract’s positive blame represents the obligations of the expression being wrapped. The negative blame represents the obligations of the macro imposing the contract—the ultimate user of expr/c. By default, the positive blame is taken as the module currently being expanded, and the negative blame is inferred from the definition site of the macro (itself inferred from the context argument), but both blame locations can be overridden.

The pos-blame and neg-blame arguments are turned into blame locations as follows:
  • If the argument is a string, it is used directly as the blame label.

  • If the argument is syntax, its source location is used to produce the blame label.

  • If the argument is a module path index, its resolved module path is used.

  • If the argument is 'from-macro, the macro is inferred from either the macro-name argument (if macro-name is an identifier) or the context argument, and the module where it is defined is used as the blame location. If neither an identifier macro-name nor a context argument is given, the location is "unknown".

  • If the argument is 'use-site, the module being expanded is used.

  • If the argument is 'unknown, the blame label is "unknown".

The macro-name argument is used to determine the macro’s binding, if it is an identifier. If expr-name is given, macro-name is also included in the contract error message. If macro-name is omitted or #f, but context is a syntax object, then macro-name is determined from context.

If expr-name is not #f, it is used in the contract’s error message to describe the expression the contract is applied to.

The context argument is used, when necessary, to infer the macro name for the negative blame party and the contract error message. The context should be either an identifier or a syntax pair with an identifier in operator position; in either case, that identifier is taken as the macro ultimately requesting the contract wrapping.

See Contracts on Macro Sub-expressions for an example.

Important: Make sure when using expr/c to use the c attribute. The expr/c syntax class does not change how pattern variables are bound; it only computes an attribute that represents the checked expression.

1.8.2 Literal Sets

Literal set containing the identifiers for fully-expanded code (Fully Expanded Programs). The set contains all of the forms listed by kernel-form-identifier-list, plus module, #%plain-module-begin, #%require, and #%provide.

Note that the literal-set uses the names #%plain-lambda and #%plain-app, not lambda and #%app.