diff options
Diffstat (limited to 'src/libexpr/primops.cc')
| -rw-r--r-- | src/libexpr/primops.cc | 25 |
1 files changed, 25 insertions, 0 deletions
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc index b833413ea..db7aa9e89 100644 --- a/src/libexpr/primops.cc +++ b/src/libexpr/primops.cc @@ -906,6 +906,30 @@ static void prim_map(EvalState & state, Value * * args, Value & v) } +/* Filter a list using a predicate; that is, return a list containing + every element from the list for which the predicate function + returns true. */ +static void prim_filter(EvalState & state, Value * * args, Value & v) +{ + state.forceFunction(*args[0]); + state.forceList(*args[1]); + + // FIXME: putting this on the stack is risky. + Value * vs[args[1]->list.length]; + unsigned int k = 0; + + for (unsigned int n = 0; n < args[1]->list.length; ++n) { + Value res; + state.callFunction(*args[0], *args[1]->list.elems[n], res); + if (state.forceBool(res)) + vs[k++] = args[1]->list.elems[n]; + } + + state.mkList(v, k); + for (unsigned int n = 0; n < k; ++n) v.list.elems[n] = vs[n]; +} + + /* Return the length of a list. This is an O(1) time operation. */ static void prim_length(EvalState & state, Value * * args, Value & v) { @@ -1120,6 +1144,7 @@ void EvalState::createBaseEnv() addPrimOp("__head", 1, prim_head); addPrimOp("__tail", 1, prim_tail); addPrimOp("map", 2, prim_map); + addPrimOp("__filter", 2, prim_filter); addPrimOp("__length", 1, prim_length); // Integer arithmetic |