I am having trouble making a type system for my EDSL in Haskell. Can
someone help? A simplified description of my problem follows.
I have three classes, A, B1, and B2. I want all instances of B1 to
be instances of A. I want all instances of B2 to be instances of A.
None of the classes have methods.
The following does NOT work, because of a duplicate instance
declaration for A:
class A a
class B1 b1
class B2 b2
instance B1 x => A x
instance B2 x => A x -- duplicate instance, won't compile
data T = T
instance B1 T
The following DOES work, but it requires that I explicitly give
instance declarations of A for all instances of B1 and B2:
class A a
class A a => B1 a
class A a => B2 a
data T = T
instance A T -- I don't want to have to specify this!
instance B1 T
Of course, this is a simplified example. In a complicated type class
hierarchy, in a program with many instances of, say B1, having to
provide instance declarations for A is laborious.
Is there a way to avoid this? How can I tell Haskell "all instances
of B1 are automatically instances of A".
--
Robin Bate Boerop
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