Haskell, parameterized datatype converting to String

Question

Here is my datatype:

data Foo a =  Value Integer
           |Boo a

and I have a function for converting the Foo datatype to String:

showFoo::Show a=> Foo a -> String
showFoo (Value n) = show n
showFoo (Boo a) = show a

For example: showFoo (Value 10) becomes: "10", but for showFoo(Boo "S"), it becomes: "\"S\"" but I need only "S".

AndrewC · Accepted Answer · 2012-10-22 11:27:55Z

4

This boils down to the behaviour of show with strings. show is designed to give machine-readable output, it's not a pretty-printer, so it puts inverted commas round all strings. That way it can tell the difference between 10 and "10" when it's reading them back in.

Your showFoo function is clearly not designed to be in the show family, since it obliterates the Value and Boo tags, so using show isn't really what you mean.

Possible solutions:

Give in, go the whole hog and derive Show.
If a is always a String, change your data type and don't use show.
Learn about type classes some more and define your own Showish class. Use -XFlexibleInstances and -XOverlappingInstances to override the instance for String, and don't use quotes.
Hack it by using init.tail.show $ a

edited Oct 22, 2012 at 11:27

answered Oct 22, 2012 at 0:39

AndrewC

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Comments

Thomas M. DuBuisson · Accepted Answer · 2012-10-21 23:02:44Z

1

That's just a result of you using GHCi and it showing the previous result. Try this in a compiled program or try running (in GHCi) putStrLn (showFoo (Boo "S")) and you will see that calling show on a string results in a single set of quotes.

answered Oct 21, 2012 at 23:02

Thomas M. DuBuisson

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3 Comments

hamid Over a year ago

I understand your point, but I can not change signature of the function (putStrLn can't be used).

Thomas M. DuBuisson Over a year ago

I sympathize with you, but I can't (or am not motivated to) help when there are artificial constraints. I suppose you could do something like filter (/='"') (show a) (or just conditionally drop " if they are the first and last characters).

Ben Over a year ago

I'm pretty sure the fundamental problem is that the OP wants show "foo" to result in "foo". I don't think not wanting to put a function Foo a -> String` into the IO monad and always write to console is an "artificial constraint" on the solution.

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