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* New language feature: with expressions.

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The expression `with E1; E2' evaluates to E2 with all bindings in
  the attribute set E1 substituted.  E.g.,

    with {x = 123;}; x

  evaluates to 123.  That is, the attribute set E1 is in scope in E2.

  This is particularly useful when importing files containing lots
  definitions.  E.g., instead of

    let {
      inherit (import ./foo.nix) a b c d e f;

      body = ... a ... f ...;
    }

  we can now say

    with import ./foo.nix;

    ... a ... f ...

  I.e., we don't have to say what variables should be brought into scope.
This commit is contained in:
Eelco Dolstra 2004-10-25 16:54:56 +00:00
parent f4d44a0026
commit 37d7abd694
7 changed files with 61 additions and 27 deletions
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9
src/libexpr/nixexpr.cc
View file

@ -296,7 +296,7 @@ void checkVarDefs(const ATermMap & defs, Expr e)
ATMatcher m;
ATerm name;
ATermList formals;
ATerm body;
ATerm with, body;
ATermList rbnds, nrbnds;
if (atMatch(m, e) >> "Var" >> name) {
@ -340,6 +340,13 @@ void checkVarDefs(const ATermMap & defs, Expr e)
}
checkVarDefs(defs2, (ATerm) rbnds);
}
else if (atMatch(m, e) >> "With" >> with >> body) {
/* We can't check the body without evaluating the definitions
(which is an arbitrary expression), so we don't do that
here but only when actually evaluating the `with'. */
checkVarDefs(defs, with);
}
else if (ATgetType(e) == AT_APPL) {
int arity = ATgetArity(ATgetAFun(e));