Monads and ArrowsMonads and Arrows may both be used for metaprogramming: programs which produce programs. However, as Pfenning and Davies write in their POPL'96 Paper ( author's mirror),
I'd like to show what this means, by example. Consider an Arrow operation
bar :: X > Bool
foo :: Arrow a => X > a Y Z foo x = if bar x then { ... } else { ... } This function has a static component which depends on the value of X. This static component involves bar, which might be a very large and computationally expensive function. The type of foo guarantees that this static component can be performed before the value of type Y is available: the decision about which Arrow values to compose via (>>>), what order to compose them in, etc, is guaranteed to be made independently of the Yvalue, and this guarantee is enforced by the type system. Now consider the monadic equivalent of the program above:
foo :: Monad m => X > (Y > m Z)
Simply by looking at the type of this function, we can't be sure that the choices about how (>>=) and return are used will be made independently of the value of type Y. For all we know, the definition might be:
baz :: Y > Bool
foo :: Monad m => X > (Y > m Z) foo x y = if baz y then { ... } else { ... } This is why Pfenning and Davies say that monadic types do not capture stage separation. ConsequencesSwierstra and Duponcheel have shown how to use this stage separation to write combinator parsers which do not rely on backtracking, making them much more efficient than monadic parsers and also capable of providing better error messages. My own work further generalizes Arrows to generalized arrows, which remove the assumption that every hostlanguage (i.e., Haskell) function may be promoted to a guestlanguage function. Without the stage separation provided by Arrows, this generalization would have been impossible. Additionally, generalized arrows are often used to produce programs in a guest language which lacks firstclass functions (for example, Verilog); the machinery of monads – which leans very heavily on firstclass functions – breaks down in this context.
