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Closures/Value capture: Difference between revisions
→{{header|Acornsoft Lisp}}
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=={{header|Acornsoft Lisp}}==
Since this Lisp is dynamically scoped and does not have any built-in closure mechanism, we have to construct one which we'll call <code>freeze</code>. (The name is inspired by the [[Pop-2]] programming languages's "frozen formals".)
('''freeze''' ''varlist lambda-expr'') finds the current values of the variables in ''varlist'' and returns a lambda-expression that is like the original except that, when called, it binds those variables to their captured values. For example, if <code>a</code>'s value is 1 and <code>b</code>'s is 2,
<syntaxhighlight lang="lisp">▼
( (lambda ((a . 1) (b . 2))▼
(list a b)) )▼
</syntaxhighlight>▼
<syntaxhighlight lang="lisp">
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(list a b c))))
</syntaxhighlight>
What does that mean? A cons (''name'' . ''value'') in a lambda-expressions's formal parameters is the syntax for a formal with a default value. The ''value'' is literally the value; it's not an expression that's evaluated. This
▲<syntaxhighlight lang="lisp">
▲( (lambda ((a . 1) (b . 2))
▲ (list a b c)) )
▲</syntaxhighlight>
calls the function represented by that lambda-expression. Since it does not give the function any arguments, <code>a</code> and <code>b</code> get their default values (which are the values captured by <code>freeze</code>).
Here is the definition of <code>freeze</code>:
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