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Use __toString when coercing sets to strings.

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For example, "${{ foo = "bar"; __toString = x: x.foo; }}" evaluates
to "bar".

With this, we can delay calling functions like mkDerivation,
buildPythonPackage, etc. until we actually need a derivation, enabling
overrides and other modifications to happen by simple attribute set
update.
This commit is contained in:
Shea Levy 2015-11-27 14:20:29 -05:00 • committed by Eelco Dolstra
parent 6298afc047
commit 14080f3e4b
2 changed files with 10 additions and 2 deletions
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10
src/libexpr/eval.cc
View file

@ -261,6 +261,7 @@ EvalState::EvalState(const Strings & _searchPath)
, sLine(symbols.create("line"))
, sColumn(symbols.create("column"))
, sFunctor(symbols.create("__functor"))
, sToString(symbols.create("__toString"))
, baseEnv(allocEnv(128))
, staticBaseEnv(false, 0)
{
@ -1389,7 +1390,14 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
}
if (v.type == tAttrs) {
Bindings::iterator i = v.attrs->find(sOutPath);
auto i = v.attrs->find(sToString);
if (i != v.attrs->end()) {
forceValue(*i->value, pos);
Value v1;
callFunction(*i->value, v, v1, pos);
return coerceToString(pos, v1, context, coerceMore, copyToStore);
}
i = v.attrs->find(sOutPath);
if (i == v.attrs->end()) throwTypeError("cannot coerce a set to a string, at %1%", pos);
return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
}