Hi Amy, On Thu, Apr 29, 2010 at 5:45 PM, Amy de Buitléir wrote:

I'm confused by the syntax this function definition I found in a tutorial:

bind :: (a → StdGen → (b,StdGen)) → (StdGen → (a,StdGen)) → (StdGen → (b,StdGen)) bind f x seed = let (x',seed') = x seed in f x' seed'

The part that confuses me is "let (x',seed') = x seed". The left side is a tuple, and the right side is two separate values, so how does the binding work? I gather from the context of the example (a tutorial on monads) that the value of x is bound to x', and the value of seed is bound to seed', so that "bind f" is a function that acts on a tuple and produces a new tuple. Is that correct?

The names and parenthesis are a bit confusing. First, the type signature bind :: (a → StdGen → (b,StdGen)) → (StdGen → (a,StdGen)) → (StdGen → (b,StdGen)) is equivalent to bind :: (a → StdGen → (b,StdGen)) → (StdGen → (a,StdGen)) → StdGen → (b,StdGen) as → associates to the right So in bind f x seed * f is a function of type (a → StdGen → (b,StdGen)), * x is a function of type (StdGen → (a,StdGen)), * and seed is a value of type StdGen. We can now see how let (x',seed') = x seed makes sense: x is applied to seed, producing a value of type (a,StdGen), a tuple, which components are assigned to the variables x' and seed'. It would perhaps have been clearer if x was called g or some other name that would make it clear that it's not of the same type as x'. Hope this helps. -- Johan