|Version 4 (modified by 11 years ago) (diff),|
Relaxed Dependency Analysis
In Haskell 98, a group of bindings is sorted into strongly-connected components, and then type-checked in dependency order (H98 s4.5.1). As each dependency group is type-checked, any binders of the group that have an explicit type signature are put in the type environment with the specified polymorphic type, and all others are monomorphic until the group is generalized (H98 s4.5.2).
data BalancedTree a = Zero a | Succ (BalancedTree (a,a)) zig :: BalancedTree a -> a zig (Zero a) = a zig (Succ t) = fst (zag t) zag (Zero a) = a zag (Succ t) = snd (zig t)
As with many operations on non-regular (or nested) types,
zag need to be polymorphic in the element type. In Haskell 98, the bindings of the two functions are interdependent, and thus constitute a single binding group. When type inference is performed on this group,
zig may be used at different types, because it has a user-supplied polymorphic signature. However,
zag may not, and the example is rejected, unless we add an explicit type signature for
However GHC, Hugs, Nhc98, and jhc follow the suggestion of Mark Jones in Typing Haskell in Haskell, that the dependency analysis should ignore references to variables that have an explicit type signature. Hence
zag does not depend on
zig, and we can infer the type
zag :: BalancedTree a -> a
and then go on to successfully check the type signature of
Dependency groups are smaller, and more programs type-check.