Unable to understand a mutual recursion

问题

I am reading Programming In Haskell, in the 8th chapter, the author gives an example of writing parsers. The full source is here: http://www.cs.nott.ac.uk/~gmh/Parsing.lhs I can't understand the following part: many permits zero or more applications of p, whereas many1 requires at least one successful application:

many        ::    Parser a → Parser [a ]
many p      =     many1 p +++ return [ ]
many1       ::    Parser a → Parser [a ]
many1 p     = do v ← p
                 vs ← many p
                 return (v : vs)

How the recursive call happens at

vs <- many p

vs is the result value of many p, but many p called many1 p, all many1 has in its definition is a do notation, and again has result value v, and vs, when does the recursive call return? Why does the following snippet can return [("123","abc")] ?

> parse (many digit) "123abc"
[("123", "abc")]

回答1:

For the last question:

> parse (many digit) "123abc"
[("123", "abc")]

Means that parsing has been successful as at least one result has been returned in the answer list. Hutton parsers always return a list - the empty list means parsing failure.

The result ("123", "abc") means that parsing has found three digits "123" and stopped at 'a' which is not a digit - so the "rest of the input" is "abc".

Note that many means "as many as possibly" not "one or more". If it were "one or more" you'd get this result instead:

[("1", "23abc"), ("12", "3abc"), ("123", "abc")]

This behaviour wouldn't be very good for deterministic parsing, though it might sometimes be needed for natural language parsing.

回答2:

The recursion stops at the v <- p line. The monadic behavior of the Parser will just propagate a [] to the end of the computation when p cannot be parsed anymore.

p >>= f =  P (\inp -> case parse p inp of
                        []        -> [] -- this line here does not call f
                        [(v,out)] -> parse (f v) out)

The second function is written in do-notation, which is just a nice syntax for the following:

many1 p = p >>= (\v -> many p >>= (\vs -> return (v : vs)))

If parsing p produces an empty list [] the function \v -> many p >>= (\vs -> return (v : vs)) will not be called, stopping the recursion.

回答3:

Let me strip this down to the barest bones to make absolutely clear why do-blocks can be misunderstood if they're read simply as imperative code. Consider this snippet:

doStuff :: Maybe Int
doStuff = do
    a <- Nothing
    doStuff

It looks like doStuff will recurse forever, after all, it's defined to do a sequence of things ending with doStuff. But the sequence of lines in a do-block is not simply a sequence of operations that is performed in order. If you're at a point in a do-block, the way the rest of the block is handled is determined by the definition of >>=. In my example, the second argument to >>= is only used if the first argument isn't Nothing. So the recursion never happens.

Something similar can happen in many different monads. Your example is just a little more complex: when there are no more ways to parse something, the stuff after the >>= is ignored.

来源：https://stackoverflow.com/questions/6080997/unable-to-understand-a-mutual-recursion