[Haskell-cafe] Show me

[Ryan Ingram]

You may find this helper function useful:

> showAp :: Show a => Int -> String -> a -> ShowS
> showAp n f x = showParen (n > ap_prec) inside
>   where
>      inside = showString f . showString " " . showsPrec (app_prec+1) x
>      ap_prec = 10  -- "precedence" of function application; higher than any infix operator.

You can use it like so:

> instance Show x => Show (Foo x) where
>   showsPrec n foo = showAp n "list_foo" (foo_list foo)

See the Haskell 98 report section 10.5 "Derived Instances: An Example":
http://www.haskell.org/onlinereport/derived.html

  -- ryan


On Tue, Aug 5, 2008 at 8:41 PM, Jonathan Cast <jonathanccast at fastmail.fm> wrote:
> On Tue, 2008-08-05 at 20:28 +0100, Andrew Coppin wrote:
>> Suppose we have the following:
>>
>>   data Foo x
>>   list_foo :: [x] -> Foo x
>>   foo_list :: Foo x -> [x]
>>
>> What would be the best way to write a Show instance?
>>
>> The thing that I came up with is
>>
>>   instance (Show x) => Show (Foo x) where
>>     show foo = "list_foo " ++ show (foo_list foo)
>>
>> But apparently you're supposed to use the strange "showsPrec" method
>> that I don't understand. So can somebody show me a correct instance
>> definition for this type please?
>
> instance Show x => Show (Foo x) where
>  showsPrec n foo = ("list_foo "++) . shows (foo_list foo)
>
> You use the n parameter if you've got an infix operator in your syntax
> and you want to put parentheses around an instance of it if n has the
> wrong value (either too high or too low, I can never remember).
>
> jcc
>
>
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