diff options
author | Eelco Dolstra <eelco.dolstra@logicblox.com> | 2012-08-13T17·46-0400 |
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committer | Eelco Dolstra <eelco.dolstra@logicblox.com> | 2012-08-13T17·46-0400 |
commit | 9c2d63084bd4f6a04210cd52b4fce054d248bc6b (patch) | |
tree | d924e30420a3ca1690b72f3b153009b1fdf2605d /src/libexpr/primops.cc | |
parent | 198d0338be7c105b6dbd707f98e0c223a8358240 (diff) |
Add a primop ‘elemAt’ to get an element from a list
Diffstat (limited to '')
-rw-r--r-- | src/libexpr/primops.cc | 24 |
1 files changed, 19 insertions, 5 deletions
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc index f5c004044135..cb09d8ff0120 100644 --- a/src/libexpr/primops.cc +++ b/src/libexpr/primops.cc @@ -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); |