Why does ghci desugar type lists and type families? Can this be selectively disabled?
ghci, haskell, types
Solution
The workaround that I know of is using :kind. For instance,
ghci> :kind (Container '[String,String,String,String,String])
Gives:
( Container '[String,String,String,String,String]) :: *
While
ghci> :kind! (Container '[String,String,String,String,String])
Will print something like this:
Container
((':)
*
[Char]
((':)
* [Char] ((':) * [Char] ((':) * [Char] ((':) * [Char] ('[] *))))))
Officially, of course, you're asking ghci a different question with `kind`, but it works. Using `undefined ::` is sort of a workaround anyhow, so I thought this might suffice.
Problem
I'm trying to make the types ghci displays for my libraries as intuitive as possible, but I'm running into a lot of difficulties when using more advanced type features. Let's say I have this code in a file: ``` {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} import GHC.TypeLits data Container (xs::[*]) = Container ``` I load it up in ghci, then I type the following command: ``` ghci> :t undefined :: Container '[String,String,String,String,String] ``` Unfortunately, ghci gives me the rather ugly looking: ``` :: Container ((':) * String ((':) * String ((':) * String ((':) * String ((':) * String ('[] *)))))) ``` ghci has removed the sugar for type level strings. Is there any way to prevent ghci from doing this and giving me just the pretty version? On a related note, lets say I create a type level `Replicate` function ``` data Nat1 = Zero | Succ Nat1 type family Replicate (n::Nat1) x :: [*] type instance Replicate Zero x = '[] type instance Replicate (Succ n) x = x ': (Replicate n x) type LotsOfStrings = Replicate (Succ (Succ (Succ (Succ (Succ Zero))))) String ``` Now, when I ask ghci for a type using `LotsOfStrings`: ``` ghci> :t undefined :: Container LotsOfStrings ``` ghci is nice and gives me the pretty result: ``` undefined :: Container LotsOfStrings ``` But if I ask for the `Replicate`d version, ``` ghci> :t undefined :: Container (Replicate (Succ (Succ (Succ (Succ (Succ Zero))))) String) ``` ghci substitutes in for the type family when it didn't do that for the type synonym: ``` :: Container ((':) * [Char] ((':) * [Char] ((':) * [Char] ((':) * [Char] ((':) * [Char] ('[] *)))))) ``` Why is ghci doing the substitution for the type family, but not the type synonym? Is there a way to control when ghci will do the substitution?