Is there any reason for not allowing:
> data Test = Test {}
in Haskell? I am generating programs where I pattern match on this
constructor: (an artificial example follows)
> main = print (case Test {} of Test {} -> True)
and am currently forced to make a special case of no fields, and
instead to write:
> data Test = Test
for ghc (2.08) and Hugs 1.4 (Jan98) and:
> data Test = Test {dummy :: Void}
for hbc (0.9999.4) (which complains about using field selection on a
constructor that was not defined with fields, which is
understandable). nhc13 fails on either as it fails on pattern
matching a construction with no fields against a pattern with no
fields. :-(
I see no reason for not allowing an empty list of fields in a data
declaration.
Cheers,
Graeme.
PS. Haskell 1.4 Report reference, Section 4.2.1:
opdecl -> data [context =>] simpletype = constrs [deriving]
...
constrs -> constr1 | ... | constrn
constr -> ...
| con { fielddecl1 , ... , fielddecln } (n>=1)
^^^^^^
