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4.1 Parsing with context
4.2 Indentation sensitivity
8.2

4 Parsers with State

So far, all of the languages we have attempted to parse have been context free, but in practice, many languages have varying amounts of context sensitivity. Parsers for such languages are often made much simpler by the addition of state that tracks the necessary context. However, megaparsack parsers can backtrack, which makes maintaining mutable state somewhat subtle: if a parser abandons a parse branch, all its state modifications must be rolled back.

To make this simpler, megaparsack provides built-in support for arbitrary, user-defined state in the form of parser parameters. Parser parameters are similar to ordinary parameters, but their values are associated with a parse context rather than with a thread. This means their values are automatically rolled back whenever the parser backtracks, and they behave predictably regardless of parser evaluation order.

4.1 Parsing with context

Suppose we have a simple language that consists of a sequence of variable declarations of the form

 

decl

 ::= 

let var = integer

where each declaration appears on a separate line. We might write a parser for such a language like this:

> (define name/p
    (map (compose1 string->symbol list->string) (many+/p letter/p)))
> (define declaration/p
    (do (string/p "let ")
        [name <- name/p]
        (string/p " = ")
        [value <- integer/p]
        (pure (cons name value))))
> (define declarations/p
    (many/p (do [decl <- declaration/p]
                (char/p #\newline)
                (pure decl))))

This definition works alright:

> (parse-string declarations/p
                (string-append "let x = 1\n"
                               "let y = 2\n"))

(success '((x . 1) (y . 2)))

However, note that it also accepts multiple declarations with the same name, which may not be desired:

> (parse-string declarations/p
                (string-append "let x = 1\n"
                               "let x = 2\n"))

(success '((x . 1) (x . 2)))

One way to prevent this is to keep track of all the declarations that we’ve parsed so far using a parser parameter:

> (define declared-names (make-parser-parameter '()))

Just like an ordinary parameter, we can read the current value of a parser parameter simply by calling it as a procedure, like (declared-names), and we can update its value by applying it to a single argument, like (declared-names new-value). However, unlike an ordinary parameter, the applying a parser parameter procedure does not directly return or update the parser parameter’s value. Instead, it returns a parser that, when executed, parses no input, but returns or updates the parser parameter’s value.

This means we can sequence reads and writes to declared-names the same way we sequence any other parser, using do:

> (define declaration/p
    (do (string/p "let ")
        [names <- (declared-names)]
        [name <- (guard/p name/p
                          (λ (name) (not (memq name names)))
                          "an unused variable name")]
        (declared-names (cons name names))
        (string/p " = ")
        [value <- integer/p]
        (pure (cons name value))))

Now duplicate definitions are rejected with a helpful error:

> (parse-result! (parse-string declarations/p
                               (string-append "let x = 1\n"
                                              "let x = 2\n")))

string:2:4: parse error

  unexpected: x

  expected: an unused variable name

4.2 Indentation sensitivity

Another useful application of parser parameters is parsing languages that are sensitive to indentation. For example, we might wish to parse a bulleted list of items, like this:

"groceries.txt"

* produce

  * apples

  * spinach

* dairy

  * milk

    * whole milk

    * buttermilk

  * cheese

    * cheddar

    * feta

To track the current indentation level, we can use a parser parameter:

> (define current-indent (make-parser-parameter 0))
> (define indentation/p
    (do [indent <- (current-indent)]
        (repeat/p indent (char/p #\space))))

This makes defining a parser for an indentation-sensitive bulleted list remarkably straightforward:

> (define tree-list/p
    (do (try/p indentation/p)
        (string/p "* ")
        [entry <- (many+/p (char-not/p #\newline))]
        (char/p #\newline)
        [indent <- (current-indent)]
        [children <- (parameterize/p ([current-indent (+ indent 2)])
                       (many/p tree-list/p))]
        (pure (list (list->string entry) children))))

The parameterize/p form works just like parameterize, but with parser parameters instead of ordinary ones. This definition of tree-list/p is enough to parse the "groceries.txt" file above:

> (define grocery-list (file->string "groceries.txt"))
> (parse-string (many/p tree-list/p) grocery-list)

(success

 '(("produce" (("apples" ()) ("spinach" ())))

   ("dairy"

    (("milk" (("whole milk" ()) ("buttermilk" ())))

     ("cheese" (("cheddar" ()) ("feta" ())))))))

Admittedly, in such a simple example, using a parser parameter is not strictly necessary. An alternative definition of tree-list/p could simply accept the indentation level as an argument:

> (define (tree-list/p indent)
    (do (try/p (repeat/p indent (char/p #\space)))
        (string/p "* ")
        [entry <- (many+/p (char-not/p #\newline))]
        (char/p #\newline)
        [children <- (many/p (tree-list/p (+ indent 2)))]
        (pure (list (list->string entry) children))))

However, in more complex parsers, this approach can require threading additional arguments through several layers of nested parsers, which is difficult to read and even more difficult to maintain. Just as ordinary parameters can help avoid threading values through many layers of nested functions, parser parameters can help avoid threading them through nested parsers.