[Haskell-cafe] Monads because of I/O?

[Olaf Klinke]

Paul,

monads were not invented because I/O could not be presented in another way 
in Haskell. Monads are way older than Haskell. It is a concept of category 
theory which was developed in the 1950s. Actually some concepts of algebra 
that are even older turn out to be monads. Take Galois theory for 
example. Once you know the pattern, you find a monad under every stone you 
turn around. It's one of the luckiest things that people like Moggi and 
Wadler realized that monads can be applied to structure programs. And 
don't blame them that monads are not composable - it is simply a 
mathematical fact. Some monads do compose with any other monad, and those 
are the monad transformers.

If you like Prolog's relational programming model so much, then you should 
play with those programs that have "no business value". Because Haskell's 
type inference algorithm, together with multi-parameter type classes and 
maybe type level natural numbers together give rise to Prolog-like 
capabilities. All the work is done by the compiler this way.

What you say about FSM is certainly true to some extent - they are well 
understood, can be generated automatically, and there is decent theory to 
reason about them. That is why this model is used in safety-sensitive 
environments such as aviation. I once applied for a position in 
verification in the automotive industry, and the interview partner told me 
that they struggle mightily with the vast state spaces of the FSMs they 
are checking.
All this speaks in favour of Haskell. The semantics is simple and 
beautiful, because it is a single-paradigm language. And because of that, 
clever people can leverage theorem provers to mathematically prove 
correctness of Haskell code. I don't know of many languages where that is 
possible. (But then, I'm not an expert on verification.)

Olaf