I'm trying to use HList and I need to annotate it but I can't figure out how to do it.
If if type the following in ghci
>:t "hello" .*. HNil
I got the following type signature :
HList ((':) * [Char] ('[] *))
Which I don't really understand. If try to use this signature I got an error
"hello" .*. HNil :: HList ((':) * [Char] ('[] *))
I got an error
<interactive>:21:15: parse error on input `:'
What should I do ?
(I've seen things using :*: so I could in theory do String :*: HNil but it can't find :*:.
The way certain type signatures are printed in older versions of ghc are quite bad. The type HList ((':) * [Char] ('[] *)) really means HList ( ([Char] :: *) ': ('[] :: *) ). But this probably isn't very clear either, so lets go through it:
[Char] :: * A list of char, whose kind (the type of a type) is *, which is the kind of things which have values. This type comes from the type of "hello"
'[] :: * The type representing the empty type level list. The type that the compiler print is actually wrong - the kind of '[] isn't *, it is [*] - the same way that you cons a and [a] with :. This type comes from the type of HNil.
': Equivalent of : but for type level lists. This comes from the type of .*.
You can't give the type signature back because it is simple invalid. But any of the following would be correct:
"hello" .*. HNil :: HList ([Char] ': '[])
"hello" .*. HNil :: HList ( ([Char] :: *) ': ('[] :: [*]))
"hello" .*. HNil :: HList '[ String ]
"hello" .*. HNil :: HList '[ (String :: *) ]
You can't write the type level list cons prefix - I would assume this is a bug in the parser:
>"hello" .*. HNil :: HList ( (':) String '[])
<interactive>:4:31: parse error on input `:'
The explicit kind annotations are unnecessary, but you can put them in anyways.
The printing was improved in ghc 7.8:
>:t "hello" .*. HNil
"hello" .*. HNil :: HList '[[Char]]