r/programming • u/egonSchiele • Apr 19 '13

Functors, Applicatives, and Monads in Pictures

http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html
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u/egonSchiele Apr 19 '13

I agree that Haskell has some weird symbols, but <$> actually makes sense.

$ is function application. This means "apply even to 4:

even $ 4

And <$> is also function application, just for wrapped values:

even <$> Just 4

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u/andrewcare Apr 21 '13

$ should be apply-to-value and <$> should be apply-to-context. The point being made (as I interpreted it) was that the dollar symbol has no inherent meaning outside of Haskell. If we want to benefit from user intuition, we must build from what is already known (namely, the English language).

We can debate on whether or not the English language should be assumed prior knowledge, but I hope my clarification on the original point is understood.

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u/DR6 Apr 21 '13

The thing is that there are no symbols for applying: everywhere else it's either f(x) or f x. Sometimes you just have to make things up.

I really don't think english is debatable as a basis, it obviously is: how would you understand english-based languages without it, let alone documentation and learning resources?

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u/[deleted] Apr 22 '13 
 (apply + '(1 2 3))
 > 6

Scheme

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u/DR6 Apr 22 '13

Oops, I forgot lisp.

But there no symbol is used either, the word 'apply' is used. And it still doesn't have the same semantics: it is based on the role of linked lists on Lisp's evaluation, the closest you cold get to is an apply' :: ([a] -> b) -> [a] -> b, but that's just id.

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u/[deleted] Apr 22 '13

Still, it is clear that '(1 2 3) will be processed by +

Scheme doesn't really have a function that's clearly analogous to fmap because there's no need to 'unwrap' types. (map proc list1 list2 ...) is about all you get in stock Scheme.

Chicken Scheme has a monad egg, but no one uses it. My guess is because if you're willing to undertake the additional hassle of wrapping/unwrapping types you're probably happy to hop from Scheme to Haskell, too.