CODEWITHSUNDEEP
haskell
user3726947
user3726947

Reputation: 521

Implementation of maximum in haskell

I was just looking up the implementation of the maximum function in haskell:

maximum :: forall a . Ord a => t a -> a
maximum = fromMaybe (errorWithoutStackTrace "maximum: empty structure") .
   getMax . foldMap (Max #. (Just :: a -> Maybe a))

Could somebody explain this piece of code to me?

In particular it is not clear to me how this part 

Max #. (Just :: a -> Maybe a)

works. What does #. mean? To me it looks like something similar to function composition but typing 

:t #.

in the REPL gives an error. Also I think the argument for foldMap should be of the form 

Monoid m => a -> m

But I can't see where the monoid structure comes from here.

Upvotes: 1

Views: 1089

Answers (1)

dfeuer
dfeuer

Reputation: 48591

#. is an operator that can make coercions easier to use.

f #. g

means

g, coerced to the type of f . g

Similarly,

f .# g

means

f, coerced to the type of f . g

In most cases, f #. g only makes sense if f is some combination of newtype constructors, newtype accessors, and fmap.

The reason for using #. in maximum us probably over-zealousness on my part. It can almost certainly be replaced with . without any problem.

Upvotes: 4

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