From c02a44183fcff7c28cfed1c84c142cc2cf80f167 Mon Sep 17 00:00:00 2001 From: Eelco Dolstra Date: Fri, 22 Sep 2006 14:46:36 +0000 Subject: [PATCH] * 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 +++++++++++++++++++++++++ tests/lang/eval-okay-list.exp | 1 + tests/lang/eval-okay-list.nix | 13 +++++++++++++ 3 files changed, 39 insertions(+) create mode 100644 tests/lang/eval-okay-list.exp create mode 100644 tests/lang/eval-okay-list.nix diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc index f08ad0464..b05082293 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); diff --git a/tests/lang/eval-okay-list.exp b/tests/lang/eval-okay-list.exp new file mode 100644 index 000000000..369d34641 --- /dev/null +++ b/tests/lang/eval-okay-list.exp @@ -0,0 +1 @@ +Str("foobarblatest") diff --git a/tests/lang/eval-okay-list.nix b/tests/lang/eval-okay-list.nix new file mode 100644 index 000000000..72a120d0d --- /dev/null +++ b/tests/lang/eval-okay-list.nix @@ -0,0 +1,13 @@ +let { + + fold = op: nul: list: + if list == [] + then nul + else op (builtins.head list) (fold op nul (builtins.tail list)); + + concat = + fold (x: y: x + y) ""; + + body = concat ["foo" "bar" "bla" "test"]; + +} \ No newline at end of file