Using the for expression in Scala for Monad operations

In Haskell, we have the syntax sugar ' do ' to help express the monad operation. We also have the corresponding syntax sugar ' for ' in Scala.

The for expression will be transformed by Scala compiler, with the following simple examples:

 for {  <- foo  <-yield (A + B)

===>

Foo.flatmap ((a) + =  {= =     {+ b  })})

So we need to implement two methods Flatmap and map.

or using the previous state Monad as an example, we add flatmap and map to the type State.

 Case class State[s, a] (runstate:s = (S, A)) (implicit m:monad[({type m[a] = state[s, A]}) #M]) {  = = B) : State[s, b] = M.bind (This, (A:A) and M.ret (f (a)))  = State[s, b]): State[s, b] = M.bind (
   
    this
    , f)}
   

Here we use an implicit parameter, and then we can use RET and bind directly.

Add a helper to simplify monad[({type m[a] = State[s, A]}) #M].ret

Def Ret[s, A] (A:A): State[s, A] = monad[({type m[a] = State[s, A]}) #M].ret (a)

Well, we can use the for expression, as in the following example:

Object Main {    import statemonad._    def main (args:array[string]) {    for  {        <-ret [int, int] (3)         <-Ret[int, Int] (4)    yield (A +b)    println (r.runstate (  1)  }}

Using the for expression in Scala for Monad operations