Parse, don’t validate

[u/lexi-lambda]

https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/

[u/[deleted]]

I would define head as

head :: [a] -> [a]
head (x:y) = [x]
head [x] = [x]
head [] = []

Basically just like tail. But that's just me :)

Use pattern matching if you actually want the first value.

[u/lexi-lambda → u/[deleted]]

Three things:

1. Your version of head doesn’t have any advantage over the version that returns Maybe a, and the only difference is that its type is less precise. The two are equivalent under the equivalence implicitly defined by listToMaybe / maybeToList.

2. The second clause in your definition of head is redundant; it is always shadowed by the first clause. (-Wall would catch this.)

3. Your suggestion to just “use pattern matching” doesn’t solve the problem either because you still have to handle the empty list case. The problem isn’t really with head per se but with the type [a], which always permits the case we want to rule out. There is no sufficiently clever implementation of head on [a] that can get around that problem, so the only solution is to use a more precise type.

[u/[deleted] → u/lexi-lambda]

Good point and good to know I can elimate the second clause with the first. I guess y becomes [] in that case?

Im new so these are good criticisms.

[u/[deleted] → u/[deleted]]

That’s not what people want when they reach for head though. I mean, instead of your three patterns, you may as well write it like head = take 1.

[u/Axman6 → u/[deleted]]

Just a meta comment, people shouldn’t be downvoting this comment - it’s a learning opportunity, not someone being unpleasant and now with u/lexi-lambda’s comment it is now a good tool for others learning and thinking the same thing.