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
<andrewcop...@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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