Dear Haskell implementors, I suggest the following small extension to the instance declaration in the language. So far -- for Haskell + glasgow-ext. I think that they are easy to implement. This is the "instance union" proposal. It is needed to write shorter several `old' instance declarations. This will make programs easier to read. It suggests the so-called inherited decl and, more general, union decl. Inherited instance decl proposal -------------------------------- Union several instance declarations with the same condition part and such that among the conclusion classes there exists some which inherits all others. Example 1. My program uses the class tower Field a Picture 1. | ERing | CRing a | Ring a / \ AddGroup a MulSemigroup a | | AddSemigroup a | \ / Set a -- "|" means that the upper inherits from the lower. Now, by the application meaning, I need to write instance (Show a, CRing a) => CRing (Pol a) where <implement operations of Set> <implement operations of AddGroup> ... <implement operations of CRing>
From the class decls it is clear to the compiler that CRing inherits all that is lower on the picture. Therefore, the conditional `instance (Show a, CRing a) =>' and 'where' is written only once. In the existing language, I need to write this conditional 6 times.
Union instance decl proposal
----------------------------
It is a generalization for inhereted decl.
instances (cond_1, ..., cond_n) -- of the type parameters a_1 ... a_m
=>
<typeTuple> (<params>) has {<conclInstList>}
where