forked from mirrors/nixpkgs
lib.list.findFirst: Make lazier
There's no need to evaluate list elements after a matching element
This commit is contained in:
parent
6996f76885
commit
9790e70150
|
@ -198,8 +198,38 @@ rec {
|
|||
default:
|
||||
# Input list
|
||||
list:
|
||||
let found = filter pred list;
|
||||
in if found == [] then default else head found;
|
||||
let
|
||||
# A naive recursive implementation would be much simpler, but
|
||||
# would also overflow the evaluator stack. We use `foldl'` as a workaround
|
||||
# because it reuses the same stack space, evaluating the function for one
|
||||
# element after another. We can't return early, so this means that we
|
||||
# sacrifice early cutoff, but that appears to be an acceptable cost. A
|
||||
# clever scheme with "exponential search" is possible, but appears over-
|
||||
# engineered for now. See https://github.com/NixOS/nixpkgs/pull/235267
|
||||
|
||||
# Invariant:
|
||||
# - if index < 0 then el == elemAt list (- index - 1) and all elements before el didn't satisfy pred
|
||||
# - if index >= 0 then pred (elemAt list index) and all elements before (elemAt list index) didn't satisfy pred
|
||||
#
|
||||
# We start with index -1 and the 0'th element of the list, which satisfies the invariant
|
||||
resultIndex = foldl' (index: el:
|
||||
if index < 0 then
|
||||
# No match yet before the current index, we need to check the element
|
||||
if pred el then
|
||||
# We have a match! Turn it into the actual index to prevent future iterations from modifying it
|
||||
- index - 1
|
||||
else
|
||||
# Still no match, update the index to the next element (we're counting down, so minus one)
|
||||
index - 1
|
||||
else
|
||||
# There's already a match, propagate the index without evaluating anything
|
||||
index
|
||||
) (-1) list;
|
||||
in
|
||||
if resultIndex < 0 then
|
||||
default
|
||||
else
|
||||
elemAt list resultIndex;
|
||||
|
||||
/* Return true if function `pred` returns true for at least one
|
||||
element of `list`.
|
||||
|
|
|
@ -554,6 +554,11 @@ runTests {
|
|||
expected = 1000000;
|
||||
};
|
||||
|
||||
testFindFirstLazy = {
|
||||
expr = findFirst (x: x == 1) 7 [ 1 (abort "list elements after the match must not be evaluated") ];
|
||||
expected = 1;
|
||||
};
|
||||
|
||||
# ATTRSETS
|
||||
|
||||
testConcatMapAttrs = {
|
||||
|
|
Loading…
Reference in a new issue