chad.scherrer:
Hello haskell-cafe,
In ghci, I tried to get info for Data.Stream.Stream:
$ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m Data.Stream Prelude Data.Stream> :i Stream data Stream a where Stream :: forall a s. (Data.Stream.Unlifted s) => !s -> Step a s -> !s -> Stream a -- Defined in Data.Stream
That's fine, and is the correct type. data Stream a = forall s. Unlifted s => Stream !(s -> Step a s) -- ^ a stepper function !s -- ^ an initial state
instance Functor Stream -- Defined in Data.Stream
This didn't seem right to me, so I asked tried this:
Prelude Data.Stream> :t Stream Stream :: (Data.Stream.Unlifted s) => (s -> Step a s) -> s -> Stream a
So that's the type of the Stream constructor, which introduces a new existentially typed Stream (the 'a').
What's going on here?
forall a s. (Data.Stream.Unlifted s) => !s -> Step a s -> !s -> Stream a and (Data.Stream.Unlifted s) => (s -> Step a s) -> s -> Stream a
One is the type, one is the constructor for the type.
are completely different, right? And really, neither one makes much sense to me. I would have expected
forall s. (Data.Stream.Unlifted s) => (s -> Step a s) -> s -> Stream a
For the constructor? This all looks right, as far as I can tell: $ ghci -fglasgow-exts Prelude> :m + Data.Stream Info about the data type: Prelude Data.Stream> :info Stream data Stream a where Stream :: forall a s. (Data.Stream.Unlifted s) => !s -> Step a s -> !s -> Stream a -- Defined in Data.Stream instance Functor Stream -- Defined in Data.Stream The type of the constructor Prelude Data.Stream> :t Stream Stream :: forall s a. (Data.Stream.Unlifted s) => (s -> Step a s) -> s -> Stream a The kind of the type: Prelude Data.Stream> :k Stream Stream :: * -> * -- Don