On 26 April 2015 at 06:10, Alexey Vagarenko wrote:

Yes, but ghci shows an error if it can't print a value, except when the value is in IO monad. Compare: Prelude> id <interactive>:6:1: No instance for (Show (a0 -> a0)) (maybe you haven't applied enough arguments to a function?) arising from a use of `print' In a stmt of an interactive GHCi command: print it Prelude>

and

Prelude> return id Prelude>

This behavior is necessary. For example, if we used any function with result of type IO (), such as writeFile, we don't want an error as we are interested in the side-effects only.

воскресенье, 26 апреля 2015 г., 5:48:55 UTC+6 пользователь Sumit Sahrawat, Maths & Computing, IIT (BHU) написал:

...

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 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 _______________________________________________ Haskell-Cafe mailing list Haskel...@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Regards

Sumit Sahrawat

-- Regards Sumit Sahrawat