[Haskell-cafe] x -> String
jochem at functor.nl
Fri Oct 16 14:29:07 EDT 2009
Andrew Coppin wrote:
> Is there any way that you can turn an arbitrary Haskell value into a
No, the only values of type
a -> String
are the constant functions and _|_.
> 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
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
> Basically, I'm writing a mutable container implementation. It can hold
> any type of data, but it would massively aid debugging if I could
> actually print out what's in it. On the other hand, I don't want to
> alter the entire program to have Show constraints everywhere just so I
> can print out some debug traces (and then alter everything back again
> afterwards once I'm done debugging).
This is not advisable, as you see.
> 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.
Jochem Berndsen | jochem at functor.nl | jochem@牛在田里.com
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