Section 4.2.1 of the Haskell Report 1.2 gives the following example of
an algebraic datatype:
data Eq a => Set a = NilSet | ConsSet a (Set a) deriving Eq
Its constructor types are
NilSet :: Forall a . Set a
ConsSet :: Forall a . Eq a => a -> Set a -> Set a
The type of NilSet has an empty context, since the context of the type
of a data constructor is restricted to those type variables free in
the component types of this data constructor. The purpose of this is
to avoid constructing a dictionary for an unspecified type.
It seems that all three Haskell compilers known to me (hbc, ghc, Yale)
infer the type
NilSet :: Forall a . Eq a => Set a
for the constructor NilSet, as seen below. Consequently, the
expression
NilSet == NilSet
leads to ambiguous overloading and produces a type error.
Was this changed after the report was published or is it a coincidence
that all three compilers forget to restrict the context? Any
clarification will be appreciated.
-Konstantin [[EMAIL PROTECTED]]
$ hbi
Welcome to interactive Haskell B. version 0.999.3 SPARC 1993 Mar 2!
Loading prelude... 1116 values, 72 types found.
Type "help;" to get help.
> data Eq a => Set a = NilSet | ConsSet a (Set a) deriving Eq;
data (Eq a) => Set a = NilSet | ConsSet a (Set a) deriving (Eq)
> NilSet == NilSet;
[55] Ambiguously overloaded tyvar(s) in class(es) Eq in show ((==) NilSet NilSet)
> whatis NilSet;
constructor NilSet :: (Eq a) => Set a
$ glhc
Enter your Haskell program, end with ^D (on a line of its own):
data Eq a => Set a = NilSet | ConsSet a (Set a) deriving Eq
v = NilSet == NilSet
main = done
"/tmp/ghc18277.hs", line 2: Ambiguous overloading:
class "Eq", type "a" (at a use of an overloaded identifier: ==)
Fail: Compilation errors found
glhc: execution of the Haskell compiler had trouble
$ haskell
Yale Haskell Y2.0-b4 CMU Common Lisp version 16f on SPARCstation
Type :? for help.
Main> data Eq a => Set a = NilSet | ConsSet a (Set a) deriving Eq
Main> :save
Main> = (NilSet == NilSet)
Main> :eval
[NON-DEFAULTABLE-AMBIGUOUS-CONTEXT] Phase error in phase TYPE:
An ambiguous context, (Eq), cannot be defaulted.
Ambiguity in call to ==
Error occurred at line 3 in file interactive.
