Re: [Haskell-cafe] x -> String

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 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 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...

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe