Polymorphism and cases - Synook/funcpy GitHub Wiki

An important part of functional languages is the ability for functions to be polymorphic. A common example of this is a functor, which can be mapped over with fmap. How fmap works for each functor is very different, however: for example, it should be the case that (fmap (* 2) (Just 3)) returns (Just 6), while (fmap (* 2) [1,2]) returns [2,4].

This is enabled in funcpy through the implementation of cases, which are a poor form of pattern matching. In essence, cases allow function aliases to "override" previous definitions of that function, but only in certain cases. For example,

fmap f v -> (case (type v "Maybe") (<something>))

would redefine fmap to be <something>, but only if the first argument of case evaluated to true. Otherwise, the original definition of fmap would be evaluated instead. This allows for fmap to be defined piecemeal for any applicable type.