|Version 1 (modified by simonpj, 3 years ago)|
The question of ambiguity in Haskell is a tricky one. This wiki page is a summary of thoughts and definitions, in the hope of gaining clarity. I'm using a wiki because it's easy to edit, and many people can contribute, even though you can't typeset nice rules.
Please edit to improve.
[Started Jan 2010.]
Suppose we have (I conflate classes Read and Show into one class Text for brevity):
class Text a where read :: String -> a show :: a -> String x :: String x = show (read "3.7")
The trouble is that there is a constraint (Text t), where t is a type variable that is otherwise unconstrained. Moreover, the type that we choose for t affects the semantics of the program. For example, if we chose t = Int then we might get x = "3", but if we choose t = Float we might get x = "3.7". This is bad: we want our type system to be coherent in the sense that every well-typed program has but a single value.
In practice, the Haskell Report, and every Haskell implementation, rejects such a program saying something like
Cannot deduce (Text t) from ()
In algorithmic terms this is very natural: we indeed have a constraint (Text t) for some unification variable t, and no way to solve it, except by searching for possible instantiations of t. So we simply refrain from trying such a search.
But in terms of the type system specification it is harder.
Problem 1: how can w