Reading the chapter on monads in real world Haskell. I came across the desugaring of the do notation, specifically when we have something like pattern <- action.
-- file: ch14/Do.hs
doNotation3 =
do pattern <- act1
act2
{- ... etc. -}
actN
The above example is desugared into:
-- file: ch14/Do.hs
translated3 =
let f pattern = do act2
{- ... etc. -}
actN
f _ = fail "..."
in act1 >>= f
I am having trouble understanding is how you can have two pattern matching cases in a let clause?
I don't understand how you can have f pattern and f _ in the same let clause. I tried looking up if you can have multiple pattern matches in a let clause, but from what I have seen, most people use a case statement to do that.
I would like some help in understanding what is actually going on here?
I am having trouble understanding is how you can have two pattern matching cases in a let clause?
Well, you can:
example :: Int -> Int
example n =
let f 0 = 1
f x = x * f (x - 1)
in f n
Remember, that's a let … in… expression, not do's let. And for all bindings in such an expression, (almost) the same rules as for your usual top-level expressions hold, e.g. you can use pattern matching.
Maybe it gets a little bit easier if you transform it to where:
example :: Int -> Int
example n = f n
where
f 0 = 1
f x = x * f (x - 1)