Eelco Dolstra <eelco.dols...@logicblox.com> writes: > Hi, > > On 12/12/12 17:15, Shea Levy wrote: > >>>> The elem library function evaluates all list elements instead of >>>> returning "true" after finding a matching element. >>> Sure about that? This seems lazy enough: >>> >>> elem = >>> builtins.elem or >>> (x: list: fold (a: bs: x == a || bs) false list); >>> >> Shouldn't the bs come first? i.e. (a: bs: bs || x == a)? > > Not if you want to check from left to right.
Sorry, but I'm not getting it yet. What I understand:
list = [0, 0, 1, 2, 3];
x = 1;
elem x list
-> a: 3, x == a: false, bs: false
-> a: 2, x == a: false; bs: false
-> a: 1, x == a: true; bs: false
-> a: 0, x == a: false; bs: true
-> a: 0, x == a: false; bs: true
Probably missing something very basic.
For completeness sake, fold from lib/lists:
fold =
if builtins ? elemAt
then op: nul: list:
let
len = length list;
fold' = n:
if n == len
then nul
else op (builtins.elemAt list n) (fold' (add n 1));
in fold' 0
else op: nul:
let fold' = list:
if list == []
then nul
else op (head list) (fold' (tail list));
in fold';
The following would not evaluate all, I'd currently claim:
elem' = x: list: if list == []
then false
else head list == x || elem' x (tail list);
--
Florian Friesdorf <f...@chaoflow.net>
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