5.16 Evaluation

The (rnrs eval (6)) library allows a program to create Scheme expressions as data at run time and evaluate them.

Procedure: eval expression environment

Evaluate expression in the specified environment and returns its value. expression must be a syntactically valid Scheme expression represented as a datum value, and environment must be an environment, which can be created using the environment procedure described below.

If the first argument to eval is determined not to be a syntactically correct expression, then eval must raise an exception with condition type &syntax. Specifically, if the first argument to eval is a definition or a splicing begin form containing a definition, it must raise an exception with condition type &syntax.

Procedure: environment import-spec …

import-spec must be a datum representing an ?import-spec (scheme library form).

The environment procedure returns an environment corresponding to import-spec.

The bindings of the environment represented by the specifier are immutable: If eval is applied to an expression that is determined to contain an assignment to one of the variables of the environment, then eval must raise an exception with a condition type &syntax.

(library (foo)
  (export)
  (import (rnrs)
          (rnrs eval))
  (write
    (eval '(let ((x 3)) x)
          (environment '(rnrs)))))      ;; writes 3

(library (foo)
  (export)
  (import (rnrs)
          (rnrs eval))
  (write
    (eval
      '(eval:car (eval:cons 2 4))
      (environment
        '(prefix (only (rnrs) car cdr cons null?)
                 eval:)))))             ;; writes 2