[Haskell-cafe] GHCi shows result of (IO a) only if (a) is in class Show

Sumit Sahrawat, Maths & Computing, IIT (BHU) sumit.sahrawat.apm13 at iitbhu.ac.in
Sat Apr 25 23:48:47 UTC 2015

If something can't be shown (converted to a string), then it can't be
printed (as a string).

On 25 April 2015 at 17:51, Daniel van den Eijkel <dvde at gmx.net> wrote:

> I wrote a parser and it took me a while to realize why GHCi suddenly did
> not show any result nor an error message anymore.
> My parsing function has type (IO Expression), Expression is in class Show.
> After changing the parser to (IO Declaration), it did not show anything
> anymore, because Declaration was not in class Show.
> When I typed (parseFile "input.txt" >>= print), I got the error message
> and understood what was going on. But for I while I was really confused
> what's happening.
> Just wanted to share this.
> Best,
> Daniel
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Sumit Sahrawat
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