[Haskell-cafe] Re: Writing guards shorthand

19 Apr 2007

      Joel Reymont  writes:
...
Support I want to infer the type given an Op that looks like
this  (incomplete):
data Op
     = Minus
     | Plus
     | Mul
     | LT
     | GT
Is there a shorthand way of bunching Minus, Plus and Mul in
a  function guard since they all result in TyNum whereas the
rest in  TyBool?
I really don't want several function clauses and neither do
I want  separate guards for every constructor.
Is there some reason why you don't want

   data Op = Aop Aop | Bop Bop
   data Aop = Minus | Plus | Mul
   data Bop = LT | GT

or similar?  I would agree that it's a shame one cannot just write

   data Op = Aop (Minus | Plus | Mul) | Bop (LT | GT)

or even, given a somewhat different type system,

   data Op = Aop | Bop
             where Aop = Minus | Plus | Mul
                   Bop = LT | GT

but it would seem reasonable to reflect the different types
of the Ops in different types in their representations.

-- 
Jón Fairbairn                                 Jon.Fairbairn@cl.cam.ac.uk