[Haskell-cafe] A question about constraints

[jean-christophe mincke]

Hello,

Given a type T, this type identifies a set of values and this set can be
deduced from the structure of type T.

i.e the type String  is the set of all possible lists of character whatever
their length.

This being said, I have the following question:

Given a type T, how is it possible to further restrict its set of values?
i.e, in the above case, how is it possible to define a type String2 whose
elements are any list of characters  whose length is <= 5.

I believe the best way to deal with that is to make String2 an  ADT. This
allows the constraint (in this case, the length) to be checked but hides the
structure of the type, thus limits the use of pattern matching (in this case
I do not want to hide the implementation of the type, so the ADT is not used
that way).

If one wants to use pattern matching, then the module must provide a way to
convert from String2 to a type whose constructors are visible (asString in
this exemple)

module A
(
String2
,mkString2
,asString
)
where

import List

data String2 =  String2 String


mkString2 l = if (length l) <= 5 then String2 l
                                 else error "length > 5"

asString (String2 l) = l

Btw, F# has found a nice solution to this problem, it allows the user to
define an "active pattern", that is, a pattern to applies on ADT:


http://blogs.msdn.com/dsyme/archive/2007/04/07/draft-paper-on-f-active-patterns.aspx

Is there another way to address that problem in Haskell?

Thank you

Regards

J-C Mincke
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