Question

On page 76 we define a function item as parser — a function that takes a String and returns [(Char, String)] or [] if failed.

On page 78 we define a function sat that takes a predicate p and “wraps” a parser construction around that

p :: (Char -> Bool) -> Parser Char
sat p = do x <- item
           if p x then return x else failure

What I don’t understand is the magic of <-? If the result of item is not empty then this operator should unwrap the list and fetch the first item from the tuple, otherwise it should produce something that won’t choke the predicate. What am I missing?

Answer 1

do x <- item
   if p x then return x else failure

This is syntactic sugar for

item >>= (\x -> if p x then return x else failure)

>>= is the monadic bind operator.

What you’re missing is: what is the definition of >>= for the Parser type? (Where in the book is the Monad instance for Parser defined? It will start instance Monad Parser where.)