Editing Scheme (programming language) (section)

===Environments and eval===
Prior to R5RS, Scheme had no standard equivalent of the <code>eval</code> procedure which is ubiquitous in other Lisps, although the first Lambda Paper had described <code>evaluate</code> as "similar to the LISP function EVAL"<ref name="lambda_paper_1"/> and the first Revised Report in 1978 replaced this with <code>enclose</code>, which took two arguments. The second, third and fourth revised reports omitted any equivalent of <code>eval</code>.

The reason for this confusion is that in Scheme with its lexical scoping the result of evaluating an expression depends on where it is evaluated. For instance, it is not clear whether the result of evaluating the following expression should be 5 or 6:<ref name="rees_1992">Jonathan Rees, [http://mumble.net/~jar/pubs/scheme-of-things/june-92-meeting.ps The Scheme of Things The June 1992 Meeting] {{Webarchive|url=https://web.archive.org/web/20110716071317/http://mumble.net/~jar/pubs/scheme-of-things/june-92-meeting.ps |date=2011-07-16}} (postscript), in Lisp Pointers, V(4), October–December 1992. 
Retrieved 2012-08-09</ref>
<syntaxhighlight lang="Scheme">
(let ((name '+))
  (let ((+ *))
    (evaluate (list name 2 3))))
</syntaxhighlight>

If it is evaluated in the outer environment, where <code>name</code> is defined, the result is the sum of the operands. If it is evaluated in the inner environment, where the symbol "+" has been bound to the value of the procedure "*", the result is the product of the two operands.

R5RS resolves this confusion by specifying three procedures that return environments and providing a procedure <code>eval</code> that takes an s-expression and an environment and evaluates the expression in the environment provided. (R5RS sec. 6.5)<ref name="r5rs"/>  R6RS extends this by providing a procedure called <code>environment</code> by which the programmer can specify exactly which objects to import into the evaluation environment.

With modern scheme (usually compatible with R5RS) to evaluate this expression, one needs to define a function <code>evaluate</code> which can look like this:

<syntaxhighlight lang="Scheme">
(define (evaluate expr)
   (eval expr (interaction-environment)))
</syntaxhighlight>

<code>interaction-environment</code> is the interpreter's global environment.