Chapter 7 | Language extensions

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.

7.1 | Streams and stream parsers

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.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.Failure` is raised. The `Stream.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 `Stream.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.

7.2 | Range patterns

In patterns, Objective Caml recognizes the form `'` `c` `'` `..` `'` `d` `'` (two character literals separated by `..`) as shorthand for the pattern
`'` `c` `'` `|` `'` `c`1 `'` `|` `'` `c`2 `'` `|` ... `|` `'` `c`n `'` `|` `'` `d` `'`
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.

7.3 | Assertion checking

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 source file name and the location of `expr` as arguments. Assertion checking can be turned off with the `-noassert` compiler option.
As a special case, `assert false` is reduced to `raise (Assert``_``failure ...)`, which is polymorphic (and is not turned off by the `-noassert` option).

7.4 | Deferred computations

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 19.14).

7.5 | Local modules

The expression `let` `module` `module-name` `=` `module-expr` `in` `expr` locally binds the module expression `module-expr` to the identifier `module-name` during the evaluation of the expression `expr`. It then returns the value of `expr`. For example:

            let remove_duplicates comparison_fun string_list =
              let module StringSet =
                Set.Make(struct type t = string
                                let compare = comparison_fun end) in
              StringSet.elements
                (List.fold_right StringSet.add string_list StringSet.empty)