This chapter describes the language features that are implemented in Objective Caml, but not described in the Objective Caml reference manual. In contrast with the fairly stable kernel language that is described in the reference manual, the extensions presented here are still experimental, and may be removed or changed in the future.
Objective Caml comprises a library type for streams (possibly infinite sequences of elements, that are evaluated on demand), and associated stream expressions, to build streams, and stream patterns, to destructure streams. Streams and stream patterns provide a natural approach to the writing of recursive-descent parsers.
Streams are presented by the following extensions to the syntactic classes of expressions:
expr:
...
| [< >]
| [< stream-component {; stream-component} >]
| parser [pattern] stream-matching
| match expr with parser [pattern] stream-matching
stream-component:
' expr
| expr
stream-matching:
stream-pattern [pattern] -> expr {| stream-pattern [pattern] -> expr}
stream-pattern:
[< >]
| [< stream-pat-comp {; stream-pat-comp [? expr]} >]
stream-pat-comp:
' pattern [when expr]
| pattern = expr
| ident
Stream expressions are bracketed by [< and >]. They
represent the concatenation of their components. The component
' expr represents the one-element stream whose element is the
value of expr. The component expr represents a
sub-stream. For instance, if both s and t are streams of integers,
then [<'1; s; t; '2>] is a stream of integers containing the
element 1, then the elements of s, then those of t, and finally
2. The empty stream is denoted by [< >].
Unlike any other kind of expressions in the language, stream expressions are submitted to lazy evaluation: the components are not evaluated when the stream is built, but only when they are accessed during stream matching. The components are evaluated once, the first time they are accessed; the following accesses reuse the value computed the first time.
Stream patterns, also bracketed by [< and >], describe
initial segments of streams. In particular, the stream pattern
[< >] matches all streams. Stream pattern components are
matched against the corresponding elements of a stream. The component
' pattern matches the corresponding stream element against the
pattern; if followed by when, the match is accepted only if the
result of the guard expression is true. The component pattern = expr applies the function denoted by expr to the current stream,
then matches the result of the function against pattern. Finally,
the component ident simply binds the identifier to the stream being
matched.
Stream matching proceeds destructively: once a component has been matched, it is discarded from the stream (by in-place modification).
Stream matching proceeds in two steps: first, a pattern is selected by matching the stream against the first components of the stream patterns; then, the following components of the selected pattern are checked against the stream. If the following components do not match, the exception Stream.Parse_error is raised. There is no backtracking here: stream matching commits to the pattern selected according to the first element. If none of the first components of the stream patterns match, the exception Stream.Parse_failure is raised. The Stream.Parse_failure exception causes the next alternative to be tried, if it occurs during the matching of the first element of a stream, before matching has committed to one pattern.
The streams hold the count of their elements discarded. The optional pattern before the first stream pattern is bound to the stream count before the matching. The one after each stream pattern (optional, too) is bound to the stream count after the matching.
The exception Parse_error has a string parameter coming from the
optional ? expr after the stream pattern components (its default
is the empty string). This expression is evaluated only in case of
error.
See Functional programming using Caml Light for a more gentle introductions to streams, and for some examples of their use in writing parsers. A more formal presentation of streams, and a discussion of alternate semantics, can be found in Parsers in ML by Michel Mauny and Daniel de Rauglaudre, in the proceedings of the 1992 ACM conference on Lisp and Functional Programming.
In patterns, Objective Caml recognizes the form
' c ' .. ' d '
(two character literals separated by ..) as shorthand for the pattern
where c1, c2, ..., cn are the characters that occur between c and d in the ASCII character set. For instance, the pattern '0'..'9' matches all characters that are digits.'c'|'c1'|'c2'|...|'cn'|'d'
Objective Caml supports the assert construct to check debugging assertions.
The expression assert expr evaluates the expression expr and
returns () if expr evaluates to true. Otherwise, the exception
Assert_failure is raised with the location of expr in the source
code as argument. As a special case, assert false is reduced to
raise (Assert_failure ...), which is polymorphic. Assertion
checking can be turned off with the -noassert compiler option.
The expression lazy expr returns a value v of type Lazy.t that
encapsulates the computation of expr. The argument expr is not
evaluated at this point in the program. Instead, its evaluation will
be performed the first time Lazy.force is applied to the value
v, returning the actual value of expr. Subsequent applications
of Lazy.force to v do not evaluate expr again.
The expression lazy expr is equivalent to
ref (Lazy.Delayed (fun () -> expr)).
For more information, see the description of module Lazy in the
standard library (section 17.12).