[Haskell-cafe] x -> String

[Ross Mellgren]

No problem, just trying to make sure the conversation stays on track :-)

-Ross

On Oct 16, 2009, at 3:26 PM, Daniel Peebles wrote:

> Whoops, sorry about that then!
>
> On Fri, Oct 16, 2009 at 2:59 PM, Ross Mellgren <rmm- 
> haskell at z.odi.ac> wrote:
>> 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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