There's not really a line between parameters and values in Haskell. Static values can just be thought of as constant, zero-arity functions. The heart of this is

There's not really a line between parameters and values in Haskell. Static values can just be thought of as constant, zero-arity functions. The heart of this is, to me, more generally thought of as polymorphism on the return type. Read is a great example of this working very well. It's, as you say, not really robust enough to work at the API or program level, but Show/Read provides a pretty quick, everywhere available serialization for many values. It's vital for debugging at the REPL. Probably the most notorious example is the newbie-killer Regex library which uses polymorphic return types to dispatch what kind of result you want from your match, such as the match index or a list of group matches. It seems really clever since you just call the match function and affect whatever kind of pattern matching you want in order to pull values out, but it has always smelled to me since it's very hard to discover the functionality — the polymorphic return types aren't opaque, but are burdensome — and seems quite a bit too magical. So I wouldn't personally go wild with return type polymorphism. It's obviously necessary in order to write things like filter, but unless the return type is simple and easily inferred by both compilers and people it can get hairy fast.