[Haskell-cafe] A function synonym is not (by default) as polymorphic as the original.

Theodore Lief Gannon tanuki at gmail.com
Wed Aug 15 03:43:30 UTC 2018

Hey, I ran into this earlier today, though it was in a where clause. :)
Luckily I already knew what to google, since there are multiple ways to
handle it and I wasn't sure which was best (I went with "just give it a

You're looking at the storied Monomorphism Restriction!


On Tue, Aug 14, 2018, 8:33 PM Jeffrey Brown <jeffbrown.the at gmail.com> wrote:

> Today I was writing a lot of Hashable instances, and found it hard to read
> code littered long lines mostly consisting of `hashWithSalt`, so I made a
> synonym:
> infixl 5 ##
> (##) = hashWithSalt
> But when I replaced all the `hashWithSalt`s with ##, I got lots of weird
> errors -- GHC kept wanting things to be Strings that were Ints, or vice
> verse. The problem vanished when I added this type signature:
> (##) :: Hashable a => Int -> a -> Int
> That's exactly the same type signature that hashWithSalt has.
> Is this expected behavior? Desirable?
> --
> Jeff Brown | Jeffrey Benjamin Brown
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