[Haskell-cafe] How to define Show [MyType] ?

Jonathan Cast jonathanccast at fastmail.fm
Thu Dec 4 17:35:09 EST 2008

On Fri, 2008-12-05 at 01:27 +0300, Dmitri O.Kondratiev wrote:
> I am trying to define instance Show[MyType] so 
> show (x:xs :: MyType) would return a single string where substrings
> corresponding to list elements will be separated by "\n".
> This would allow pretty printing of MyType list in several lines
> instead of one, as default Show does for lists.
> For example:
> data ShipInfo = Ship {
>       name :: String,
>       kind :: String,
>       canons :: Int
> } deriving Show
> s1 = Ship {name ="HMS Fly", kind = "sloop", canons=16} 
> s2 = Ship {name ="HMS Surprise", kind = "frigate", canons=42} 
> -- Yet when I try to define:
> instance (Show ShipInfo) => Show [ShipInfo] where
>     show (x:xs) = "<" ++ show x ++ ">" ++ show xs

The context on this is borked: you already know Show ShipInfo, so you
don't need to assume it here.

> -- I get this error:
>     Illegal instance declaration for `Show [ShipInfo]'
>         (The instance type must be of form (T a b c)
>          where T is not a synonym, and a,b,c are distinct type
> variables)

Read this error again.  Your instance is for the type `[] ShipInfo',
which does not have the form GHC listed for you.  The instance in
GHC.Show (don't import it from there!  Import it from Prelude, instead)
is for a type of the form `[] a', which does have that form.

Now, in this case, you don't need to define Show ([ShipInfo]), because
the instance for Show [a] already does what you want; you just need to
define an explicit instance for Show ShipInfo and over-ride the showList

If you really, really wanted to define Show [ShipInfo], then putting

{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}

at the beginning of your file would work.  At the cost of using
overlapping instances, of course.


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