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Allow external code using libnixexpr to add types

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Code that links to libnixexpr (e.g. plugins loaded with importNative, or
nix-exec) may want to provide custom value types and operations on
values of those types. For example, nix-exec is currently using sets
where a custom IO value type would be more appropriate. This commit
provides a generic hook for such types in the form of tExternal and the
ExternalBase virtual class, which contains all functions necessary for
libnixexpr's type-polymorphic functions (e.g. `showType`) to be
implemented.
This commit is contained in:
Shea Levy 2014-11-30 13:16:19 -05:00
parent 5f04da905f
commit 320659b0cd
5 changed files with 108 additions and 0 deletions
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33
src/libexpr/eval.cc
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@ -104,6 +104,9 @@ static void printValue(std::ostream & str, std::set<const Value *> & active, con
case tPrimOpApp:
str << "<PRIMOP-APP>";
break;
case tExternal:
str << *v.external;
break;
default:
throw Error("invalid value");
}
@ -136,6 +139,7 @@ string showType(const Value & v)
case tBlackhole: return "a black hole";
case tPrimOp: return "a built-in function";
case tPrimOpApp: return "a partially applied built-in function";
case tExternal: return v.external->showType();
}
abort();
}
@ -1314,6 +1318,9 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
}
if (v.type == tExternal)
return v.external->coerceToString(pos, context, coerceMore, copyToStore);
if (coerceMore) {
/* Note that `false' is represented as an empty string for
@ -1434,6 +1441,9 @@ bool EvalState::eqValues(Value & v1, Value & v2)
case tPrimOpApp:
return false;
case tExternal:
return *v1.external == *v2.external;
default:
throwEvalError("cannot compare %1% with %2%", showType(v1), showType(v2));
}
@ -1575,6 +1585,11 @@ size_t valueSize(Value & v)
sz += doValue(*v.primOpApp.left);
sz += doValue(*v.primOpApp.right);
break;
case tExternal:
if (seen.find(v.external) != seen.end()) break;
seen.insert(v.external);
sz += v.external->valueSize(seen);
break;
default:
;
}
@ -1601,4 +1616,22 @@ size_t valueSize(Value & v)
}
string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore)
{
throw TypeError(format("cannot coerce %1% to a string, at %2%") %
showType() % pos);
}
bool ExternalValueBase::operator==(const ExternalValueBase & b)
{
return false;
}
std::ostream & operator << (std::ostream & str, ExternalValueBase & v) {
return v.print(str);
}
}