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Add primop genList

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This can be used to implement functions like ‘imap’ (or for that
matter, ‘map’) without the quadratic complexity incurred by calling
‘++’ repeatedly.
This commit is contained in:
Eelco Dolstra 2015-07-28 17:27:32 +02:00
parent f3dda728a4
commit 50807f3dd5
3 changed files with 41 additions and 3 deletions
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18
doc/manual/expressions/builtins.xml
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@ -364,6 +364,24 @@ builtins.fromJSON ''{"x": [1, 2, 3], "y": null}''
</varlistentry>
<varlistentry><term><function>builtins.genList</function>
<replaceable>generator</replaceable> <replaceable>length</replaceable></term>
<listitem><para>Generate list of size
<replaceable>length</replaceable>, with each element
<replaceable>i></replaceable> equal to the value returned by
<replaceable>generator</replaceable> <literal>i</literal>. For
example,
<programlisting>
builtins.genList (x: x * x) 5
</programlisting>
returns the list <literal>[ 0 1 4 9 16 ]</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.getAttr</function>
<replaceable>s</replaceable> <replaceable>set</replaceable></term>

20
src/libexpr/primops.cc
View file

@ -1364,6 +1364,25 @@ static void prim_all(EvalState & state, const Pos & pos, Value * * args, Value &
}
/* Apply a function to every element of a list. */
static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
state.forceFunction(*args[0], pos);
auto len = state.forceInt(*args[1], pos);
if (len < 0)
throw EvalError(format("cannot create list of size %1%, at %2%") % len % pos);
state.mkList(v, len);
for (unsigned int n = 0; n < len; ++n) {
Value * arg = state.allocValue();
mkInt(*arg, n);
mkApp(*(v.listElems()[n] = state.allocValue()), *args[0], *arg);
}
}
/*************************************************************
* Integer arithmetic
*************************************************************/
@ -1759,6 +1778,7 @@ void EvalState::createBaseEnv()
addPrimOp("__foldl'", 3, prim_foldlStrict);
addPrimOp("__any", 2, prim_any);
addPrimOp("__all", 2, prim_all);
addPrimOp("__genList", 2, prim_genList);
// Integer arithmetic
addPrimOp("__add", 2, prim_add);

4
tests/lang/lib.nix
View file

@ -54,8 +54,8 @@ rec {
const = x: y: x;
range = first: last:
if builtins.lessThan last first
if first > last
then []
else [first] ++ range (builtins.add first 1) last;
else genList (n: first + n) (last - first + 1);
}