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POWhy does ghci desugar type lists and type families? Can this be selectively disabled?
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copied!<p>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.</p> <p>Let's say I have this code in a file:</p> <pre><code>{-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} import GHC.TypeLits data Container (xs::[*]) = Container </code></pre> <p>I load it up in ghci, then I type the following command:</p> <pre><code>ghci> :t undefined :: Container '[String,String,String,String,String] </code></pre> <p>Unfortunately, ghci gives me the rather ugly looking:</p> <pre><code>:: Container ((':) * String ((':) * String ((':) * String ((':) * String ((':) * String ('[] *)))))) </code></pre> <p>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?</p> <hr> <p>On a related note, lets say I create a type level <code>Replicate</code> function </p> <pre><code>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 </code></pre> <p>Now, when I ask ghci for a type using <code>LotsOfStrings</code>:</p> <pre><code>ghci> :t undefined :: Container LotsOfStrings </code></pre> <p>ghci is nice and gives me the pretty result:</p> <pre><code>undefined :: Container LotsOfStrings </code></pre> <p>But if I ask for the <code>Replicate</code>d version,</p> <pre><code>ghci> :t undefined :: Container (Replicate (Succ (Succ (Succ (Succ (Succ Zero))))) String) </code></pre> <p>ghci substitutes in for the type family when it didn't do that for the type synonym:</p> <pre><code>:: Container ((':) * [Char] ((':) * [Char] ((':) * [Char] ((':) * [Char] ((':) * [Char] ('[] *)))))) </code></pre> <p>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?</p>

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