[Haskell-cafe] x -> String

Daniel Peebles pumpkingod at gmail.com
Fri Oct 16 14:46:23 EDT 2009

My GHCi can't do that :o

I just wrote data A = B | C and loaded the file into GHCi. Typing B gives me:

    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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