Hello
I am going through Walder's "Monads for Functional Programming" document.
On page 9, there is a piece of code that provides the definition of return
and bind for the State Monad
I have pasted two variations of the code. The first one matches the
definition given in the book.
The second one is what I came up with.
-- State Monad variation
-- Retain the eval function as such
-- Modify M , unit, * definitions
type M a = State->(a, State)
type State = Int
unit :: a -> M a
unit a = (\e->(a,e))
star :: M a -> (a -> M b) -> M b
m `star` k = (\x -> let (a, aState) = m x in
let (b, bState) = k a aState in
(b, bState))
-- State Monad variation -- Retain the eval function as such -- Modify M ,
unit, * definitions type M a = State->(a, State) type State = Int unit :: a
-> M a unit a = (\e->(a,e)) star :: M a -> (a -> M b) -> M b m `star` k =
(\x -> let (a, aState) = m x (b, bState) = k a aState in (b, bState))
In the first definition of star, there are two let statements, while I found
that one let statement would do the work anyway, as done in the second
defintion of start
I am not able to understand if there is any semantic implication in this
syntatic variation of let.
Please help.
Thanks
--
Regards
Lakshmi Narasimhan T V
