[Haskell-beginners] Show and showList

[Brandon Allbery]

On Fri, Jan 23, 2015 at 8:21 PM, Animesh Saxena <animeshsaxena at icloud.com>
wrote:

> It's the same problem now transfered to showList which will be declared
> differently for a list of int's or a list of chars, but still it outputs
> them differently as can be seen from the above code. How it is able to
> differentiate without throwing up the overlap error?


The key is that showList is defined on e.g. Int or Char, not [Int] or
[Char]. If we tried to do the "obvious" thing for a list, we'd define

    instance Show a => Show [a] where ...

but then we have an overlap any time we try to define a different handler
for a specific list of things (in this case, instance Show [Char] would be
the overlapping instance we'd want, to produce the String representation).
We avoid this by building the representation for lists into the
representation for single items (thus, a showList method); we can provide a
default implementation of that method in the Show class to do the usual
brackets-and-commas representation, using the mechanism intended to allow
optimized versions of typeclass methods for particular types, and then
provide a specific implementation for instance Show Char without invoking
overlap.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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