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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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28
src/libexpr/eval.cc
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@ -230,7 +230,7 @@ Expr evalExpr2(EvalState & state, Expr e)
try {
return evalExpr(state, substArgs(e4, formals, e2));
} catch (Error & e) {
throw Error(format("while evaluating function at %1%:\n%2%")
throw Error(format("while evaluating the function at %1%:\n%2%")
% showPos(pos) % e.msg());
}
}
@ -241,7 +241,7 @@ Expr evalExpr2(EvalState & state, Expr e)
subs.set(name, e2);
return evalExpr(state, substitute(subs, e4));
} catch (Error & e) {
throw Error(format("while evaluating function at %1%:\n%2%")
throw Error(format("while evaluating the function at %1%:\n%2%")
% showPos(pos) % e.msg());
}
}
@ -258,7 +258,7 @@ Expr evalExpr2(EvalState & state, Expr e)
try {
return evalExpr(state, a);
} catch (Error & e) {
throw Error(format("while evaluating attribute `%1%' at %2%:\n%3%")
throw Error(format("while evaluating the attribute `%1%' at %2%:\n%3%")
% s1 % showPos(pos) % e.msg());
}
}
@ -283,6 +283,26 @@ Expr evalExpr2(EvalState & state, Expr e)
return evalExpr(state, e2);
}
/* Withs. */
if (atMatch(m, e) >> "With" >> e1 >> e2 >> pos) {
ATermMap attrs;
try {
e1 = evalExpr(state, e1);
queryAllAttrs(e1, attrs);
} catch (Error & e) {
throw Error(format("while evaluating the `with' definitions at %1%:\n%2%")
% showPos(pos) % e.msg());
}
try {
e2 = substitute(attrs, e2);
checkVarDefs(state.primOps, e2);
return evalExpr(state, e2);
} catch (Error & e) {
throw Error(format("while evaluating the `with' body at %1%:\n%2%")
% showPos(pos) % e.msg());
}
}
/* Generic equality. */
if (atMatch(m, e) >> "OpEq" >> e1 >> e2)
return makeBool(evalExpr(state, e1) == evalExpr(state, e2));
@ -357,7 +377,7 @@ Expr evalFile(EvalState & state, const Path & path)
try {
return evalExpr(state, e);
} catch (Error & e) {
throw Error(format("while evaluating file `%1%':\n%2%")
throw Error(format("while evaluating the file `%1%':\n%2%")
% path % e.msg());
}
}