[Haskell-cafe] Typeclass default implementation in subclasses

[Andy Gimblett]

Hi all,

This email is literate Haskell.  I have a question about default
implementations of typeclasses.

 > {-# LANGUAGE TypeSynonymInstances #-}

 > module Thing where

 > import Text.PrettyPrint.HughesPJ

Let say I want to pretty-print some values, enclosed in double quotes.
The natural thing to do (within the HughesPJ pretty-printing
framework, anyway - and that's where I am in this problem's wider
context) is:

 > ppQuote :: Show a => a -> Doc
 > ppQuote = doubleQuotes . text . show

Now, this works nicely for (say) Int:

 > x :: Int
 > x = 1

*Thing> ppQuote x
"1"

But less nicely for String and Char, because their Show instances
already insert double/single quotes respectively:

 > y :: String
 > y = "hello"
 > z :: Char
 > z = 'a'

*Thing> ppQuote y
""hello""
*Thing> ppQuote z
"'a'"

I don't want this.  I'd like them to be "hello" and "a" respectively.

So I thought I'd create a typeclass, whose default implementation is
as above...

 > class (Show a) => Quotable a where
 >   quote :: a -> Doc
 >   quote = ppQuote

... but with specialised instances for String and Char (the former
seems to need the TypeSynonymInstances extension?):

 > instance Quotable String where
 >   quote = text . show -- don't need the doubleQuotes call for String
 > instance Quotable Char where
 >   quote c = quote [c] -- just lift it to String

Unfortunately, while this works great for String and Char...

*Thing> quote y
"hello"
*Thing> quote z
"a"

... the "default implementation" mechanism doesn't work as I'd
expect/hope:

*Thing> quote x

<interactive>:1:0:
     No instance for (Quotable Int)
       arising from a use of `quote' at <interactive>:1:0-6
     Possible fix: add an instance declaration for (Quotable Int)
     In the expression: quote x
     In the definition of `it': it = quote x

What I would _like_ would be for the compiler to say "OK, the Quotable
class depends on the Show class, and Int is an instance of Show so Int
is also an instance of Quotable, having the default implementation
(since there isn't a specialised one for it)" - but clearly it
doesn't.

Please can someone tell me why this doesn't happen, and if there is a
way of making it happen?  Also, if there's a more sensible way of
attacking this whole problem, I'd be curious to hear it.

I should perhaps add that this isn't a huge problem for me, because my
instances will in practice tend to be String and Char anyway, and one
can of course add Quotable instances for anything else easily enough -
but I'm curious now I've come this far.  :-)

Many thanks!

-Andy