* 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).
This commit is contained in:
Eelco Dolstra 2006-09-22 14:46:36 +00:00
parent 8a1ab709a4
commit c02a44183f
3 changed files with 39 additions and 0 deletions

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@ -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);

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@ -0,0 +1 @@
Str("foobarblatest")

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@ -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"];
}