> I would like to avoid using show all the time for printing strings e.g.
>
> > val = "the sum of 2 and 2 is "++(show $ 2 + 2)++" whenever."
>
> I would prefer to type something like:
>
> > val = "the sum of 2 and 2 is "./(2+2)./" whenever."
> > -- i can' find a better haskell compatible operator
>
> I can't simply "show" the arguments of (./) because showing strings adds
> quotation marks which I don't want in this context.
>
> So I tried creating my own Stringable class:
> > class Stringable a where
> > toString::a -> String
>
> > (./) :: (Stringable a,Stringable b)=> a->b->String
> > x./y = (toString x)++(toString y)
Nice idea. I polished the code somewhat ...
===============================================================================
> infixr 0 &
What about `&' for catenation?
> toString :: (Stringable a) => a -> String
> toString a = toStrings a ""
> class (Show a) => Stringable a where
> toStrings :: a -> ShowS
> toStringList :: [a] -> ShowS
>
> toStrings = shows
> toStringList [] = showString "[]"
> toStringList (a : as) = showChar '[' . toStrings a . showl as
> where showl [] = showChar ']'
> showl (a : as) = showString ", " . toStrings a . showl as
The class `Stringable' uses the `ShowS' mechanism to avoid quadratic
time behavior and employs the standard trick to allow overlapping
instances (Eric Meijer has written a short note about that topic):
[Char] and [a] should be treated differently.
> instance Stringable Char where
> toStringList s = showString s
This instance declaration print strings as they are ...
> instance Stringable Int
>
> instance Stringable Integer
>
> instance Stringable a => Stringable [a] where
> toStrings = toStringList
>
> instance Stringable ShowS where
> toStrings = id
This instance declaration is necessary to make `&' useable. Note that
this is not (Standard) Haskell but works only with Hugs 1.3c (and
probably with GHC's next release).
> (&) :: (Stringable a, Stringable b) => a -> b -> ShowS
> a & b = toStrings a . toStrings b
Note that `&' yields `ShowS' and not `String'.
> val = "the sum of 2 and 2 is " & (2 + 2 :: Int) & " whenever."
Furthermore note that `val' has type `ShowS'. If quadratic time
behaviour is not a problem (does not occur?) you can safely omit the
`Stringable ShowS' instance and change `&' to `toString a ++ toString b'.
> render :: (Stringable a) => a -> IO ()
> render a = putStr (toString a)
`render' is quite flexible:
? render val
the sum of 2 and 2 is 4 whenever.
? render (toString "aaa")
aaa
? render "aaa"
aaa
? render (toStrings "aaa")
aaa
===============================================================================
> Is there any way to convince Haskell to just resolve these numbers to
> SOMETHING by default? Then I can just declare that type an instance of
> Stringable.
Unfortunately not. I did not succeed in persuading SPJ ;-). See
http://www.cs.chalmers.se/~rjmh/Haskell/Messages/Display.cgi?id=445
HTH, Ralf
