From c02a44183fcff7c28cfed1c84c142cc2cf80f167 Mon Sep 17 00:00:00 2001 From: Eelco Dolstra Date: Fri, 22 Sep 2006 14:46:36 +0000 Subject: * Builtin functions `head' and `tail' to return the head and tail of list. Useful for lots of things, such as implementing a fold function (see NIX-30, example is in tests/lang/eval-okay-list.nix). --- src/libexpr/primops.cc | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) (limited to 'src/libexpr') diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc index f08ad0464feb..b05082293fb7 100644 --- a/src/libexpr/primops.cc +++ b/src/libexpr/primops.cc @@ -673,6 +673,27 @@ static Expr primAbort(EvalState & state, const ATermVector & args) } +/* Return the first element of a list. */ +static Expr primHead(EvalState & state, const ATermVector & args) +{ + ATermList list = evalList(state, args[0]); + if (ATisEmpty(list)) + throw Error("`head' called on an empty list"); + return evalExpr(state, ATgetFirst(list)); +} + + +/* Return a list consisting of everything but the the first element of + a list. */ +static Expr primTail(EvalState & state, const ATermVector & args) +{ + ATermList list = evalList(state, args[0]); + if (ATisEmpty(list)) + throw Error("`tail' called on an empty list"); + return makeList(ATgetNext(list)); +} + + /* Apply a function to every element of a list. */ static Expr primMap(EvalState & state, const ATermVector & args) { @@ -703,6 +724,7 @@ static Expr primCurrentTime(EvalState & state, const ATermVector & args) } +/* Dynamic version of the `.' operator. */ static Expr primGetAttr(EvalState & state, const ATermVector & args) { string attr = evalString(state, args[0]); @@ -710,6 +732,7 @@ static Expr primGetAttr(EvalState & state, const ATermVector & args) } +/* Dynamic version of the `?' operator. */ static Expr primHasAttr(EvalState & state, const ATermVector & args) { string attr = evalString(state, args[0]); @@ -761,6 +784,8 @@ void EvalState::addPrimOps() addPrimOp("isNull", 1, primIsNull); addPrimOp("dependencyClosure", 1, primDependencyClosure); addPrimOp("abort", 1, primAbort); + addPrimOp("__head", 1, primHead); + addPrimOp("__tail", 1, primTail); addPrimOp("map", 2, primMap); addPrimOp("__getAttr", 2, primGetAttr); -- cgit 1.4.1