Alberto G. Corona wrote:

I think that a non-non recursive let could be not compatible with the pure nature of Haskell.

I have seen this sentiment before. It is quite a mis-understanding. In fact, the opposite is true. One may say that Haskell is not quite pure _because_ it has recursive let. Let's take pure the simply-typed lambda-calculus, or System F, or System Fomega. Or the Calculus of Inductive Constructions. These calculi are pure in the sense that the result of evaluation of each expression does not depend on the evaluation strategy. One can use call-by-name, call-by-need, call-by-value, pick the next redex at random or some other evaluation strategy -- and the result will be just the same. Although the simply-typed lambda-calculus is quite limited in its expressiveness, already System F is quite powerful (e.g., allowing for the list library), to say nothing of CIC. In all these systems, the non-recursive let let x = e1 in e2 is merely the syntactic sugar for (\x. e2) e1 OTH, the recursive let is not expressible. (Incidentally, although System F and above express self-application (\x.x x), a fix-point combinator is not typeable.) Adding the recursive let introduces general recursion and hence the dependence on the evaluation strategy. There are a few people who say non-termination is an effect. The language with non-termination is no longer pure.