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Add a primop ‘elemAt’ to get an element from a list

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This commit is contained in:
Eelco Dolstra 2012-08-13 13:46:42 -04:00
parent 198d0338be
commit 9c2d63084b
1 changed files with 19 additions and 5 deletions
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24
src/libexpr/primops.cc
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@ -868,14 +868,27 @@ static void prim_isList(EvalState & state, Value * * args, Value & v)
}
static void elemAt(EvalState & state, Value & list, int n, Value & v)
{
state.forceList(list);
if (n < 0 || n >= list.list.length)
throw Error(format("list index %1% is out of bounds") % n);
state.forceValue(*list.list.elems[n]);
v = *list.list.elems[n];
}
/* Return the n-1'th element of a list. */
static void prim_elemAt(EvalState & state, Value * * args, Value & v)
{
elemAt(state, *args[0], state.forceInt(*args[1]), v);
}
/* Return the first element of a list. */
static void prim_head(EvalState & state, Value * * args, Value & v)
{
state.forceList(*args[0]);
if (args[0]->list.length == 0)
throw Error("`head' called on an empty list");
state.forceValue(*args[0]->list.elems[0]);
v = *args[0]->list.elems[0];
elemAt(state, *args[0], 0, v);
}
@ -1164,6 +1177,7 @@ void EvalState::createBaseEnv()
// Lists
addPrimOp("__isList", 1, prim_isList);
addPrimOp("__elemAt", 2, prim_elemAt);
addPrimOp("__head", 1, prim_head);
addPrimOp("__tail", 1, prim_tail);
addPrimOp("map", 2, prim_map);