Is there a way to express the following Hugs (Gofer) Haskell extension in GHC?
From the Hugs User's Manual, "Section 7.3.5 Restricted type synonyms"
type T a1 ... am = rhs in f1, ..., fn where T is a new type constructor name and rhs is a type expression typically involving some of the (distinct) type variables a1, ..., am. The major difference with a normal type synonym definition is that
Hugs supports the use of restricted type synonyms, first introduced in Gofer, and similar to the mechanisms for defining abstract datatypes that were provided in several earlier languages. The purpose of a restricted type synonym is to restrict the expansion of a type synonym to a particular set of functions. Outside of the selected group of functions, the synonym constructor behaves like a standard datatype. More precisely, a restricted type synonym definition is a top level declaration of the form: the expansion of the type synonym can only be used within the binding group of one of the functions f1, ..., fn (all of which must be defined by top-level definitions in the module containing the restricted type synonym definition). In the definition of any other value, T is treated as if it had been introduced by a definition of the form:
data T a1 ... am = ...
For my intended application, this last line, the type synonym is treated as if it had been introduced with "data" is of key importance. Here are some code snippets, taken from the paper "Using catamorphisms, subtypes and monad transformers for writing modular functional interpretors" by Luc Duponcheel (Nov. 1995) further explain what I to express:
data Sum x y = L x | R y
type E x = Sum (E1 x) (E2 x) in fmapE, phiE
fmapE :: (x -> y) -> (E x -> E y) fmapE g = (L . fmap1 g) <+> (R . fmap2 g)
phiE :: E Int -> Int phiE = phi1 <+> phi2
data Rec1 f = In (f (Rec1 f))
type Expr = Rec1 E
instance Functor E where fmap = fmapE
Trying to compile this with GHC (after removing the "in fmapE, phiE"), generates the (expected) error:
Type synonym `E' should have 1 argument, but has been given 0 In the type: Rec1 E While checking the RHS of a type synonym declaration `Expr'
for the line: type Expr = Rec1 E and, if I replace that line with: type Expr x = Rec1 E x I get the following error message:
Couldn't match `*' against `k -> k1' Expected kind: * Inferred kind: k -> k1 When checking kinds in `Rec1 E x' In the type synonym declaration for `Expr'
and, more problems are anticipated for:
instance Functor E where fmap = fmapE
Thanks in advance for all help, Ed Komp komp@ittc.ku.edu