[Haskell-cafe] x -> String

[Ross Mellgren]

Andrew has mentioned the debugger several times, NOT the interactive  
REPL. That is, using :-commands to inspect values.

-Ross

On Oct 16, 2009, at 2:46 PM, Daniel Peebles wrote:

> My GHCi can't do that :o
>
> I just wrote data A = B | C and loaded the file into GHCi. Typing B  
> gives me:
>
> <interactive>:1:0:
>    No instance for (Show A)
>      arising from a use of `print' at <interactive>:1:0
>    Possible fix: add an instance declaration for (Show A)
>    In a stmt of a 'do' expression: print it
>
> The error also gives an idea of what GHCi is doing behind the scenes:
> it's just calling print, which has a Show constraint.
>
> On Fri, Oct 16, 2009 at 2:40 PM, Andrew Coppin
> <andrewcoppin at btinternet.com> wrote:
>> Jochem Berndsen wrote:
>>>
>>>> I rephrase: There *is* a way to turn arbitrary values into  
>>>> strings. I
>>>> know there is, because the GHCi debugger *does* it. The question  
>>>> is,
>>>> does anybody know of an /easy/ way to do this?
>>>>
>>>
>>> No. GHCi does not always do this:
>>>
>>> Prelude Data.Ratio> let plus1 = (+1)
>>> Prelude Data.Ratio> plus1
>>>
>>> <interactive>:1:0:
>>>    No instance for (Show (a -> a))
>>>      arising from a use of `print' at <interactive>:1:0-4
>>>    Possible fix: add an instance declaration for (Show (a -> a))
>>>    In a stmt of a 'do' expression: print it
>>> Prelude Data.Ratio>
>>>
>>
>> The GHCi *debugger* can print out even values for which no Show  
>> instance
>> exists. (But yes, it fails to print anything interesting for function
>> types... It works for ADTs that don't have Show though.)
>>
>>>> Anybody know of a way to do this? (As it happens, the values I'm  
>>>> testing
>>>> with are all Showable anyway, but the type checker doesn't know  
>>>> that...)
>>>>
>>>
>>> What is the problem with adding a function
>>> showMyContainer :: (Show a) => Container a -> String
>>> ?
>>> In this case you can show your container (for debugging purposes),  
>>> but
>>> only if you have Showable elements in your container.
>>>
>>
>> This could plausibly work...
>>
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