Added Hashed type for caching the 'hash' function result.

[mvv]

Supposed to save time spent on calculating hashes in lookups and other operations in unordered-containers.

[hvr]

See discussion in #22

[mvv]

Hm, one benefit of the polymorphic version is that you don't have to write Eq, Ord, Hashable instances and hashed, unhashed functions by yourself. Just write

type MyText = Hashed Text
instance IsString MyText where
  fromString = hashed . Text.pack

and you are good to go. And even if you want a monomorphic type, the polymorphic version instances can still be reused via newtype deriving:

newtype MyText = MyText (Hashed Text) deriving (Eq, Ord, Hashable)

[tibbe]

@mvv I'm willing to add (some version of) this if you have a real need where you can show a speed-up.

[mvv]

I'm going to use this in the "renamer" stage of a compiler I'm writing (The parser represents symbols as Hashed Text and later all the symbols in the AST need to be replaced with glorified unique integers. The walker that does the job uses a HashMap (Hashed Text) Symbol to do the mapping) . I need more time to come up with a benchmark (the source code would probably need to be quite long to show a speed-up)..

[phadej]

should this one be defaultHashWithSalt to benefit from Hashed property?

[tibbe]

We should define both hash and hashWithSalt. The latter could use defaultHashWithSalt.

[andrewthad]

I'm currently using something similar to optimize a syntax tree. It would be nice to have this functionality in hashable. The Eq instance looks a little dubious:

instance Eq a => Eq (Hashed a) where
  Hashed a _ == Hashed b _ = a == b

Since the purpose of a hash is to speed up equality tests, I think it should be:

instance Eq a => Eq (Hashed a) where
  Hashed a ha == Hashed b hb = if ha == hb then a == b else False

Or more succinctly:

instance Eq a => Eq (Hashed a) where
  Hashed a ha == Hashed b hb = (ha == hb) && (a == b)

The Ord instance could benefit similarly.

[tibbe]

@andrewthad the point is to avoid hashing, but we should definitely use the cached hash to speed up the negative case for equality checking as well.

This pull request seems abandoned. Do you want to pick it up?

[andrewthad]

Yeah, I'll commandeer this.

[andrewthad]

New PR is at #121