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Chapter 7  Language extensions

This chapter describes language extensions and convenience features that are implemented in Objective Caml, but not described in the Objective Caml reference manual.

7.1  Integer literals for types int32, int64 and nativeint

int32-literal::= integer-literal l  
int64-literal::= integer-literal L  
nativeint-literal::= integer-literal n

An integer literal can be followed by one of the letters l, L or n to indicate that this integer has type int32, int64 or nativeint respectively, instead of the default type int for integer literals. The library modules Int32[Int32], Int64[Int64] and Nativeint[Nativeint] provide operations on these integer types.

7.2  Streams and stream parsers

The syntax for streams and stream parsers is no longer part of the Objective Caml language, but available through a Camlp4 syntax extension. See the Camlp4 reference manual for more information. Support for basic operations on streams is still available through the Stream[Stream] module of the standard library. Objective Caml programs that use the stream parser syntax should be compiled with the -pp camlp4o option to ocamlc and ocamlopt. For interactive use, run ocaml and issue the #load "camlp4o.cma";; command.

7.3  Recursive definitions of values

As mentioned in section 6.7.1, the let rec binding construct, in addition to the definition of recursive functions, also supports a certain class of recursive definitions of non-functional values, such as

let rec name1 = 1 ::  name2 and  name2 = 2 ::  name1 in  expr

which binds name1 to the cyclic list 1::2::1::2::…, and name2 to the cyclic list 2::1::2::1::…Informally, the class of accepted definitions consists of those definitions where the defined names occur only inside function bodies or as argument to a data constructor.

More precisely, consider the expression:

let rec name1 =  expr1 andand  namen =  exprn in  expr

It will be accepted if each one of expr1 …  exprn is statically constructive with respect to name1 …  namen and not immediately linked to any of name1 …  namen

An expression e is said to be statically constructive with respect to the variables name1 …  namen if at least one of the following conditions is true:

An expression e is said to be immediately linked to the variable name in the following cases:

7.4  Range patterns

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

' c ' | ' c1 ' | ' c2 ' || ' cn ' | ' 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.

7.5  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).

7.6  Lazy evaluation

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. Applications of Lazy.force may be implicit through pattern matching.

The pattern lazy pattern matches values v of type Lazy.t, provided pattern matches the result of forcing v with Lazy.force. A successful match of a pattern containing lazy sub-patterns forces the corresponding parts of the value being matched, even those that imply no test such as lazy value-name or lazy _. Matching a value with a pattern-matching where some patterns contain lazy sub-patterns may imply forcing parts of the value, even when the pattern selected in the end has no lazy sub-pattern.

For more information, see the description of module Lazy in the standard library (see Module Lazy).

7.7  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
            (List.fold_right StringSet.add string_list StringSet.empty)

7.8  Recursive modules

definition::= ...  
  module rec module-name :  module-type =  module-expr   { and module-name:  module-type =  module-expr }  
specification::= ...  
  module rec module-name :  module-type  { and module-name:  module-type }

Recursive module definitions, introduced by the 'module rec' …'and' … construction, generalize regular module definitions module module-name =  module-expr and module specifications module module-name :  module-type by allowing the defining module-expr and the module-type to refer recursively to the module identifiers being defined. A typical example of a recursive module definition is:

    module rec A : sig
                     type t = Leaf of string | Node of ASet.t
                     val compare: t -> t -> int
                 = struct
                     type t = Leaf of string | Node of ASet.t
                     let compare t1 t2 =
                       match (t1, t2) with
                         (Leaf s1, Leaf s2) -> s1 s2
                       | (Leaf _, Node _) -> 1
                       | (Node _, Leaf _) -> -1
                       | (Node n1, Node n2) -> n1 n2
        and ASet : Set.S with type elt = A.t
                 = Set.Make(A)

It can be given the following specification:

    module rec A : sig
                     type t = Leaf of string | Node of ASet.t
                     val compare: t -> t -> int
        and ASet : Set.S with type elt = A.t

This is an experimental extension of Objective Caml: the class of recursive definitions accepted, as well as its dynamic semantics are not final and subject to change in future releases.

Currently, the compiler requires that all dependency cycles between the recursively-defined module identifiers go through at least one “safe” module. A module is “safe” if all value definitions that it contains have function types typexpr1 ->  typexpr2. Evaluation of a recursive module definition proceeds by building initial values for the safe modules involved, binding all (functional) values to fun _ -> raise Undefined_recursive_module. The defining module expressions are then evaluated, and the initial values for the safe modules are replaced by the values thus computed. If a function component of a safe module is applied during this computation (which corresponds to an ill-founded recursive definition), the Undefined_recursive_module exception is raised.

7.9  Private types

Private type declarations in module signatures, of the form type t = private ..., enable libraries to reveal some, but not all aspects of the implementation of a type to clients of the library. In this respect, they strike a middle ground between abstract type declarations, where no information is revealed on the type implementation, and data type definitions and type abbreviations, where all aspects of the type implementation are publicized. Private type declarations come in three flavors: for variant and record types (section 7.9.1), for type abbreviations (section 7.9.2), and for row types (section 7.9.3).

7.9.1  Private variant and record types

type-representation::= ...  
  = private constr-decl  { | constr-decl }  
  = private { field-decl  { ; field-decl } }

Values of a variant or record type declared private can be de-structured normally in pattern-matching or via the expr .  field notation for record accesses. However, values of these types cannot be constructed directly by constructor application or record construction. Moreover, assignment on a mutable field of a private record type is not allowed.

The typical use of private types is in the export signature of a module, to ensure that construction of values of the private type always go through the functions provided by the module, while still allowing pattern-matching outside the defining module. For example:

        module M : sig
                     type t = private A | B of int
                     val a : t
                     val b : int -> t
                 = struct
                     type t = A | B of int
                     let a = A
                     let b n = assert (n > 0); B n

Here, the private declaration ensures that in any value of type M.t, the argument to the B constructor is always a positive integer.

With respect to the variance of their parameters, private types are handled like abstract types. That is, if a private type has parameters, their variance is the one explicitly given by prefixing the parameter by a `+' or a `-', it is invariant otherwise.

7.9.2  Private type abbreviations

type-equation::= ...  
  = private typexpr

Unlike a regular type abbreviation, a private type abbreviation declares a type that is distinct from its implementation type typexpr. However, coercions from the type to typexpr are permitted. Moreover, the compiler “knows” the implementation type and can take advantage of this knowledge to perform type-directed optimizations. For ambiguity reasons, typexpr cannot be an object or polymorphic variant type, but a similar behaviour can be obtained through private row types.

The following example uses a private type abbreviation to define a module of nonnegative integers:

        module N : sig
                     type t = private int
                     val of_int: int -> t
                     val to_int: t -> int
                 = struct
                     type t = int
                     let of_int n = assert (n >= 0); n
                     let to_int n = n

The type N.t is incompatible with int, ensuring that nonnegative integers and regular integers are not confused. However, if x has type N.t, the coercion (x :> int) is legal and returns the underlying integer, just like N.to_int x. Deep coercions are also supported: if l has type N.t list, the coercion (l :> int list) returns the list of underlying integers, like N.to_int l but without copying the list l.

Note that the coercion (expr :> typexpr) is actually an abbreviated form, and will only work in presence of private abbreviations if both the type of expr and typexpr contain no type variables. If this is not the case, you must use the full form (expr : typ_e :> typexpr) where typ_e is the expected type of expr. Concretely, this would be (x : N.t :> int) and (l : N.t list :> int list) for the above examples.

7.9.3  Private row types

type-equation::= ...  
  = private typexpr

Private row types are type abbreviations where part of the structure of the type is left abstract. Concretely typexpr in the above should denote either an object type or a polymorphic variant type, with some possibility of refinement left. If the private declaration is used in an interface, the corresponding implementation may either provide a ground instance, or a refined private type.

   module M : sig type c = private < x : int; .. > val o : c end =
       class c = object method x = 3 method y = 2 end
       let o = new c

This declaration does more than hiding the y method, it also makes the type c incompatible with any other closed object type, meaning that only o will be of type c. In that respect it behaves similarly to private record types. But private row types are more flexible with respect to incremental refinement. This feature can be used in combination with functors.

   module F(X : sig type c = private < x : int; .. > end) =
       let get_x (o : X.c) = o#x
   module G(X : sig type c = private < x : int; y : int; .. > end) =
       include F(X)
       let get_y (o : X.c) = o#y

Polymorphic variant types can be refined in two ways, either to allow the addition of new constructors, or to allow the disparition of declared constructors. The second case corresponds to private variant types (one cannot create a value of the private type), while the first case requires default cases in pattern-matching to handle addition.

   type t = [ `A of int | `B of bool ]
   type u = private [< t > `A ]
   type v = private [> t ]

With type u, it is possible to create values of the form (`A n), but not (`B b). With type v, construction is not restricted but pattern-matching must have a default case.

Like for abstract and private types, the variance of type parameters is not inferred, and must be given explicitly.

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