[Haskell-cafe] Deriving Show for non-regular data types

[Mark Laws]

On Thu, Mar 12, 2015 at 12:25 PM, adam vogt <vogt.adam at gmail.com> wrote:
> This combination seems to work:
>
> instance (Show a, Show1 d) => Show (d a) where
>     showsPrec = showsPrec1
>
> deriving instance (Show (d a), Show1 d)
>   => Show (SimpleCatDeque d a)
>
> But it needs overlapping instances. I don't see another way to express
>
>  (Show (d a), Show (d (d a)), Show (d (d (d a))), ... )
>
> in a way that ghc will lazily evaluate the ...

Hi Adam and David,

Thanks for the tips.  This works, but there's another issue now:

> :load SimpleCatenableDeque.hs BankersDeque.hs
> import BankersDeque
> cons 1 (cons 2 empty ++ (cons 3 (cons 7 (cons 123 empty)))) ++ (cons 4 (cons 5 (empty) ++ (cons 6 (cons (-1) (cons 99 empty) ++ (cons 72 empty))) ++ (cons 44 empty ++ (cons 7 (cons 8 (cons 9 (cons 10 (cons 11 empty))))))) ++ cons 9 (cons 10 (cons 123 (empty :: SimpleCatDeque BankersDeque Int) ++ (cons 83 empty))))

<interactive>:28:1-315:
    No instance for (Show1 BankersDeque) arising from a use of 'print'
    In a stmt of an interactive GHCi command: print it

What would the necessary instance look like for BankersDeque?

Thanks,
Mark

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