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Add primop genList

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This can be used to implement functions like ‘imap’ (or for that
matter, ‘map’) without the quadratic complexity incurred by calling
‘++’ repeatedly.
This commit is contained in:
Eelco Dolstra 2015-07-28 17:27:32 +02:00
parent f3dda728a4
commit 50807f3dd5
3 changed files with 41 additions and 3 deletions
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20
src/libexpr/primops.cc
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@ -1364,6 +1364,25 @@ static void prim_all(EvalState & state, const Pos & pos, Value * * args, Value &
}
/* Apply a function to every element of a list. */
static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
state.forceFunction(*args[0], pos);
auto len = state.forceInt(*args[1], pos);
if (len < 0)
throw EvalError(format("cannot create list of size %1%, at %2%") % len % pos);
state.mkList(v, len);
for (unsigned int n = 0; n < len; ++n) {
Value * arg = state.allocValue();
mkInt(*arg, n);
mkApp(*(v.listElems()[n] = state.allocValue()), *args[0], *arg);
}
}
/*************************************************************
* Integer arithmetic
*************************************************************/
@ -1759,6 +1778,7 @@ void EvalState::createBaseEnv()
addPrimOp("__foldl'", 3, prim_foldlStrict);
addPrimOp("__any", 2, prim_any);
addPrimOp("__all", 2, prim_all);
addPrimOp("__genList", 2, prim_genList);
// Integer arithmetic
addPrimOp("__add", 2, prim_add);