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\DOC leftbin
\TYPE {leftbin : ('a -> 'b * 'c) -> ('c -> 'd * 'a) -> ('d -> 'b -> 'b -> 'b) -> string -> 'a -> 'b * 'c}
\SYNOPSIS
Parses iterated left-associated binary operator.
\DESCRIBE
If {p} is a parser for ``items'' of some kind, {s} is a parser for some
``separator'', {c} is a `constructor' function taking an element as parsed by
{s} and two other elements as parsed by {p} and giving a new such element, and
{e} is an error message, then {leftbin p s c e} will parse an iterated sequence
of items by {p} and separated by something parsed with {s}. It will repeatedly
apply the constructor function {c} to compose these elements into one,
associating to the left. For example, the input:
{
<p1> <s1> <p2> <s2> <p3> <s3> <p4>
}
meaning successive segments {pi} that are parsed by {p} and {sj} that are
parsed by {s}, will result in
{
c (c s2 (c s1 p1 p2) p3) p4
}
\FAILURE
The call {leftbin p s c e} never fails, though the resulting parser may.
\COMMENTS
This is one of a suite of combinators for manipulating ``parsers''. A parser is
simply a function whose OCaml type is some instance of
{:('a)list -> 'b * ('a)list}. The function should take a list of objects of
type {:'a} (e.g. characters or tokens), parse as much of it as possible from
left to right, and return a pair consisting of the object derived from parsing
(e.g. a term or a special syntax tree) and the list of elements that were not
processed.
\SEEALSO
++, ||, >>, a, atleast, elistof, finished, fix, listof, many, nothing,
possibly, rightbin, some.
\ENDDOC
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