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FAQ - Core language

Basic types

Is it possible to do computations with arbritrary precision arithmetics?
OCaml and Caml Light both provide a library that handles exact arithmetic computation for rational numbers. The library is called Num in OCaml and camlnum in Caml Light.
Operations on big numbers gets the suffix /: addition is thus +/. You build big numbers using conversion from (small) integers or character strings. For printing in the toplevel, a custom printer can be used. An example under OCaml is given below.
$ ocaml nums.cma

open Num;;
open Format;;
let print_num ff n = fprintf ff "%s" (string_of_num n);;
print_num : Format.formatter -> num -> unit = <fun>;
#install_printer print_num;;
num_of_string "2/3";;
- : Num.num = 2/3
let n = num_of_string "1/3" +/ num_of_string "2/3";;
n : Num.num = 1
let rec fact n =
 if n <= 0 then (num_of_int 1) else num_of_int n */ (fact (n - 1));;
fact : int -> Num.num = <fun>;
fact 100;;
- : Num.num =
9332621544394415268169923885626670049071596826438162146859296389521759999322991
5608941463976156518286253697920827223758251185210916864000000000000000000000000

Data structures

My array is modified, and I don't know why
This is due to the physical sharing of two arrays that you missed. In Caml there are no implicit array copying. If you give two names to the same array, every modification on one array will be visible to the other:
let v = Array.make 3 0;;
val v : int array = [|0; 0; 0|]
let w = v;;
val w : int array = [|0; 0; 0|]
w.(0) <- 4;;
- : unit = ()
v;;
- : int array = [|4; 0; 0|]
The physical sharing effect also applies to elements stored in vectors: if these elements are also vectors, the sharing of these vectors implies that modifying one of these elements modifies the others (see also the entry below).
How to define multidimensional arrays?
The only way is to define an array whose elements are arrays themselves (Caml arrays are unidimensional, they modelize mathematical vectors). The naive way to define multidimensional arrays is bogus: the result is not right because there is some unexpected physical sharing between the lines of the new array (see also previous entry):
let m = Array.make 2 (Array.make 3 0);;
m : int array array = [|[|0; 0; 0|]; [|0; 0; 0|]|]
m.(0).(0) <- 1;;
- : unit = ()
m;;
- : int array array = [|[|1; 0; 0|]; [|1; 0; 0|]|]
The allocation of a new array has two phases. First, the initial value is computed; then this value is written in each element of the new array. That's why the line which is allocated by Array.make 3 0 is unique and physically shared by all the lines of the array m.
The solution is to use the make_matrix primitive that builds the matrix with all elements equal to the initial value provided. Alternatively, write the program that allocates a new line for each line of your matrix. For instance:
let matrix n m init =
  let result = Array.make n (Array.make m init) in
    for i = 1 to n - 1 do
      result.(i) <- Array.make m init
    done;
    result;;
matrix : int -> int -> 'a -> 'a array array = <fun>
In the same vein, the copy_vect primitive gives strange results, when applied to matrices: you need to write a function that explicitly copies each line of the matrix at hand:
let copy_matrix m =
  let l = Array.length m in
    if l = 0 then m else
      let result = Array.make l m.(0) in
        for i = 0 to l - 1 do
           result.(i) <- Array.copy m.(i)
        done;
        result

Types definitions

How to define an enumerated type?
An enumerated type is a sum type with only constants. For instance, a type with 3 constants:
type color = Blue | White | Red;;
type color = Blue | White | Red
Blue;;
- : color = Blue
The names Blue, White and Red are the constructors of the color type. One can define functions on this type by pattern matching:
let string_of_color = function
    Blue -> "blue"
  | White -> "white"
  | Red -> "red";;
How to share a label between two different record types?
When you define two types sharing a label name, the last defined type hides the labels of the first type. For instance:
type point_3d = {x : float; y : float; z : float};;
type point_2d = {x : float; y : float};;

{x = 10.; y = 20.; z = 30.};;
The record field label z belongs to the type point_3d
but is here mixed with labels of type point_2d
The simplest way to overcome this problem is simply ... to use different names! For instance
type point3d = {x3d : float; y3d : float; z3d : float};;
type point2d = {x2d : float; y2d : float};;
With OCaml, one can propose two others solutions. First, it is possible to use modules to define the two types in different name spaces:
module D3 = struct
 type point = {x : float; y : float; z : float}
end;;

module D2 = struct
 type point = {x : float; y : float}
end;;
This way labels can be fully qualified as D3.x D2.x:
{D3.x = 10.; D3.y = 20.; D3.z = 30.};;
- : D3.point = {D3.x = 10.000000; D3.y = 20.000000; D3.z = 30.000000}
{D2.x = 10.; D2.y = 20.};;
- : D2.point = {D2.x = 10.000000; D2.y = 20.000000}
You can also use objects that provide overloading on method names:
class point_3d ~x ~y ~z = object
  method x : float = x
  method y : float = y
  method z : float = z
end;;

class point_2d ~x ~y = object
  method x : float = x
  method y : float = y
end;;
Note that objects provide you more than overloading: you can define truly polymorphic functions, working on both point_3d and point_2d, and you can even coerce a point_3d to a point_2d.
How to define two sum types that share constructor names?
Generally speaking you cannot. As for all other names, you must use distinct name constructors. However, you can define the two types in two different name spaces, i.e. into two different modules. As for labels discussed above, you obtain constructors that can be qualified by their module names. With OCaml you can alternatively use polymorphic variants, i.e. constructors that are, in some sense, predefined, since they are not defined by a type definition. For instance:
type ids = [ `Name | `Val ];;
type person = [ `Name of string ];;
let (f : person -> string) = function `Name s -> s;;
val f : person -> string = <fun>
let (is_name : ids -> bool) = function `Name -> true | _ -> false;;
val is_name : ids -> bool = <fun>

Functions and procedures

How to define a function?
In Caml, the syntax to define functions is close to the mathematical usage: the definition is introduced by the keyword let, followed by the name of the function and its arguments; then the formula that computes the image of the argument is written after an = sign.
let successor (n) = n + 1;;
successor : int -> int = <fun>
In fact, parens surrounding the argument may be omitted, so we generally write:
let successor n = n + 1;;
successor : int -> int = <fun>
How to define a recursive function?
You need to explicitly tell that you want to define a recursive function: use “let rec” instead of “let”. For instance:
let rec fact n =
  if n = 0 then 1 else n * fact (n - 1);;
fact : int -> int = <fun>
let rec fib n =
  if n <= 1 then n else fib (n - 1) + fib (n - 2);;
fib : int -> int = <fun>
Functions may be mutually recursive:
let rec odd n =
  if n = 0 then true
  else if n = 1 then false else even (n - 1)
and even n =
  if n = 0 then false
  else if n = 1 then true else odd (n - 1);;
odd : int -> bool = <fun>
even : int -> bool = <fun>
How to apply a function?
Functions are applied as in mathematics: write the function's name, followed by its argument enclosed in parens: f (x). In practice, parens are omitted in case of constants or identifiers: we write fib 2 instead of fib (2), and fact x instead of fact (x).
When the argument of a function is more complex than just an identifier, you must enclose this argument between parentheses. In particular you need parens when the argument is a negative constant number: to apply f to -1 you must write f (-1) and not f -1 that is syntactically similar to f - 1 (hence it is a subtraction, not an application).
How to define a procedure?
Recall that procedures are commands that produce an effect (for instance printing something on the terminal or writing some memory location), but have no mathematically meaningful result.
In Caml, there is no special treatment of procedures: they are just considered as special cases of functions that return the special “meaningless” value (). For instance, the print_string primitive that prints a character string on the terminal, just returns () as a way of indicating that its job has been properly completed.
Procedures that do not need any meaningful argument, get () as dummy argument. For instance, the print_newline procedure, that outputs a newline on the terminal, gets no meaningful argument: it has type unit -> unit.
Procedures with argument are defined exactly as ordinary functions. For instance:
let message s = print_string s; print_newline();;
message : string -> unit = <fun>
message "Hello world!";;
Hello world!
- : unit = ()
How to define a procedure/function that takes no argument?
Note that it is impossible to define a procedure without any argument at all: its definition would imply to execute it, and there would be no way to call it afterwards. In the following fragment double_newline is bound to (), and its further evaluation never produces carriage returns as may be erroneously expected by the user.
let double_newline = print_newline(); print_newline();;
@
@
double_newline : unit = ()
double_newline;;
- : unit = ()
The correct definition and usage of this procedure is:
let double_newline () = print_newline(); print_newline();;
double_newline : unit -> unit = <fun>
double_newline;;
- : unit -> unit = <fun>
double_newline ();;
@
@
- : unit = ()
How to define a function with more than one argument?
Just write the list of successive arguments when defining the function. For instance:
let sum x y = x + y;;
sum : int -> int -> int = <fun>
then gives the actual arguments in the same order when applying the function:
sum 1 2;;
- : int = 3
These functions are named “curried” functions, as opposed to functions with tuples as argument:
let sum' (x, y) = x + y;;
sum' : int * int -> int = <fun>
sum' (1, 2);;
- : int = 3
How to define a function that has several results?
You can define a function that return a pair or a tuple:
let div_mod x y = (x quo y, x mod y);;
div_mod : int -> int -> int * int = <fun>
div_mod 15 7;;
- : int * int = 2, 1
What is an “anonymous function”?
You may use functions that have no names: we call them functional values or anonymous functions. A functional value is introduced by the keyword function, followed by its argument, then an arrow -> and the function body. For instance:
function x -> x + 1;;
- : int -> int = fun
(function x -> x + 1) 2;;
- : int = 3
What is the difference between fun and function?
Functions are usually introduced by the keyword function. Each parameter is introduced by its own function construct. For instance, the construct:
function x -> function y -> ...
defines a function with two parameters x and y. Functions that use pattern-matching are also introduced by the keyword function.
The keyword fun introduces curried functions (with several successive arguments). For instance:
fun x y -> ...
introduces a function with two arguments x and y as to the previous one.
My function is never applied
This is probably due to a missing argument: since Caml is a functional programming language, there is no error when you evaluate a function with missing arguments: in this case, a functional value is returned, but the function is evidently not applied. Example: if you evaluate print_newline without argument, there is no error, but nothing happens. The compiler issues a warning in case of a blatant misuse.
#print_newline;;
- : unit -> unit
#print_newline ();;
@
- : unit = ()

Pattern matching

How to do nested pattern matching?
You imperatively need to enclose between parens a pattern matching which is written inside another pattern matching. In effect, the internal pattern matching “catches” all the pattern matching clauses that are written after it. For instance:
let f = function
 | 0 -> match ... with | a -> ... | b -> ...
 | 1 -> ...
 | 2 -> ...;;
is parsed as
let f = function
 | 0 ->
     match ... with
     | a -> ...
     | b -> ...
     | 1 -> ...
     | 2 -> ...;;
This error may occur for every syntactic construct that involves pattern matching: function, match .. with and try ... with. The usual trick is to enclose inner pattern matchings with begin and end. One write:
let f = function
 | 0 ->
     begin match ... with
     | a -> ...
     | b -> ...
     end
 | 1 -> ...
 | 2 -> ...;;

Exceptions

Typing

Error message: a type is not compatible with itself
You may obtain the message: This expression has type “some type” but is used with type “the same some type”. This may occur very often when using the interactive system.
The reason is that two types with the same name have been defined the compiler does not confuse the two types, but the types are evidently written the same. Consider for instance:
type t = T of int
type t = T of int
let x = T 1;;
val x : t = T 1
type t = T of int;;
type t = T of int
let incr = function T x -> T (x+1);;
val incr : t -> t = <fun>
incr x;;
This expression has type t but is here used with type t
This phenomenon appears when you load many times the same file into the interactive system, since each reloading redefines the types. The solution is to quit your interactive system and reload your files in a new session.
A function obtained through partial application is not polymorphic enough
The more common case to get a ``not polymorphic enough'' definition is when defining a function via partial application of a general polymorphic function. In Caml polymorphism is introduced only through the “let” construct, and results from application are weakly polymorph; hence the function resulting from the application is not polymorph. In this case, you recover a fully polymorphic definition by clearly exhibiting the functionality to the type-checker : define the function with an explicit functional abstraction, that is, add a function construct or an extra parameter (this rewriting is known as eta-expansion):
let map_id = List.map (function x -> x) (* Result is weakly polymorphic *)
val map_id : '_a list -> '_a list = <fun>
map_id [1;2]
- : int list = [1;2]
map_id (* No longer polymorphic *)
- : int list -> int list = <fun>
let map_id' l = List.map (function x -> x) l
val map_id' : 'a list -> 'a list = <fun>
map_id' [1;2]
- : int list = [1;2]
map_id' (* Still fully polymorphic *)
- : 'a list -> 'a list = <fun>
The two definitions are semantically equivalent, and the new one can be assigned a polymorphic type scheme, since it is no more a function application.
The type of this expression contains type variables that cannot be generalized
This message appears when the Caml compiler tries to compile a function or a value which is monorphic, but for which some types have not been completely inferred. Some types variables are left in the type, which are are called “weak” (and are displayed by an underscore: '_a); they will disappear thanks to type inference as soon as enough informations will be given.
let r = ref []
val r : '_a list ref = {contents = []}
let f = List.map (fun x -> x)
val f : '_a list -> '_a list = <fun>
Since the expression mentionned in the error message cannot be compiled as is, two cases must be envisioned:
  • The expression can really not be turned into a polymorphic expression, as in r above. You must use an explicit type annotation, in order to turn it into something completely monomorphic.
  • The expression can be transformed into something polymorphic through rewriting some part of the code (for example using eta-expansion) as in the case of f.
How to write a function with polymorphic arguments?
In ML, an argument of a function cannot be polymorphic inside the body of the function; hence the following typing:
let f (g : 'a -> 'a) x y = g x, g y
val f : ('a -> 'a) -> 'a -> 'a -> 'a * 'a = <fun>
The function is not as polymorphic as we could have hoped.
Nevertheless, in OCaml it is possible to use first-order polymorphism. For this, you can use either records or objects; in the case of records, you need to declare the type before using it in the function.
let f (o : <g : 'a. 'a -> 'a>) x y = o#g x, o#g y
val f : < g : 'a. 'a -> 'a > -> 'b -> 'c -> 'b * 'c = <fun>
type id = { g : 'a. 'a -> 'a; }
type id = { g : 'a. 'a -> 'a; }
let f r x y = r.g x, r.g y
val f : id -> 'a -> 'b -> 'a * 'b = <fun>

Intput/output

Why some printing material is mixed up and does not appear in the right order?
If you use printing functions of the format module, you might not mix printing commands from format with printing commands from the basic I/O system. In effect, the material printed by functions from the format module is delayed (stored into the pretty-printing queue) in order to find out the proper line breaking to perform with the material at hand. By contrast low level output is performed with no more buffering than usual I/O buffering.
print_string "before";
Format.print_string "MIDDLE";
print_string "after";;
beforeafterMIDDLE- : unit = ()
To avoid this kind of problems you should not mix printing orders from format and basic printing commands; that's the reason why when using functions from the format module, it is considered good programming habit to open format globally in order to completely mask low level printing functions by the high level printing functions provided by format.