Standard ML and Objective Caml, Side by Side

This page gives a quick side by side comparison of program fragments in the two ML dialects Standard ML ('97 revision) and Objective Caml (version 3.12). It is primarily targetted at people who need to convert code between the two dialects. Where suitable we also mention common extensions to SML, or recent extensions of Ocaml. The comparison does not cover features that do not have an appropriate counter part in the sibling dialect (e.g. Ocaml's object sublanguage, SML's user-defined operator fixity, or advanced library issues).

The first section is an interaction with the respective toplevel system, in order to show the built-in types. The rest just consists of example expressions and definitions. Keywords and other reserved symbols are type-set in blue.

Literals

SML Ocaml
- 3;
> val it = 3 : int
# 3;;
- : int = 3
- 3.141;
> val it = 3.141 : real
# 3.141;;
- : float = 3.141
- "Hello world";
> val it = "Hello world" : string
# "Hello world";;
- : string = "Hello world"
- #"J";
> val it = #"J" : char
# 'J';;
- : char = 'J'
- true;
> val it = true : bool
# true;;
- : bool = true
- ();
> val it = () : unit
# ();;
- : unit = ()
- (3, true, "hi");
> val it = (3, true, "hi") : int * bool * string
# (3, true, "hi");;
- : int * bool * string = 3, true, "hi"
- [1, 2, 3];
> val it = [1, 2, 3] : int list
# [1; 2; 3];;
- : int list = [1; 2; 3]
- #[1, 2, 3];
> val it = #[1, 2, 3] : int vector
Standard does not have vector literals but most implementations support them – use library functions otherwise
Does not have vectors – use arrays
Does not have array literals – use library functions # [|1; 2; 3|];;
- : int array = [|1; 2; 3|]

Expressions

SML Ocaml
~3*(1+7) div 2 mod 3
-3*(1+7)/2 mod 3
~1.0/2.0 + 1.9*x
-1.0 /. 2.0 +. 1.9 *. x
a orelse b andalso c


a || b && c
or (deprecated)
a or b & c

Functions

SML Ocaml
fn f => fn x => fn y => f(x,y)


fun f -> fun x -> fun y -> f (x,y)
or
fun f x y -> f (x,y)
fn 0 => 0
 | n => 1
function 0 -> 0
       | n -> 1
f o g
fun x -> f (g x)

map SOME xs
Does not have first-class constructors – use function instead, e.g.
map (fun x -> Some x) xs

map #2 triples
map #lab records
Does not have first-class selectors – use function instead, e.g.
map (fun (_,x,_) -> x) triples
map (fun x -> x.lab) records
f (inputLine stdIn) (inputLine stdIn)


Evaluation order is undefined for application – use let, e.g.
let line1 = read_line () in
let line2 = read_line () in
f line1 line2

Control Flow

SML Ocaml
if 3 > 2 then "X" else "Y"
if 3 > 2 then "X" else "Y"
if 3 > 2 then print "hello" else ()

if 3 > 2 then print_string "hello"
Note: expression has to have type unit
while true do
   print "X"

while true do
   print_string "X"
done
Does not have for loops – use recursion or while for i = 1 to 10 do
   print_endline "Hello"
done
(print "Hello ";
 print "world")








print_string "Hello ";
print_string "world"
or
(print_string "Hello ";
 print_string "world")
or
begin
   print_string "Hello ";
   print_string "world"
end

Value Declarations

SML Ocaml
val name = expr
let name = expr
fun f x y = expr
let f x y = expr
val rec fib = fn n =>
   if n < 2
   then n
   else fib(n-1) + fib(n-2)
or
fun fib n =
   if n < 2
   then n
   else fib(n-1) + fib(n-2)
let rec fib = fun n ->
   if n < 2
   then n
   else fib (n-1) + fib (n-2)
or
let rec fib n =
   if n < 2
   then n
   else fib (n-1) + fib (n-2)

Type Declarations

SML Ocaml
type t = int -> bool
type t = int -> bool
type ('a,'b) assoc_list = ('a * 'b) list
type ('a,'b) assoc_list = ('a * 'b) list
datatype 'a option = NONE | SOME of 'a
type 'a option = None | Some of 'a
datatype t = A of int | B of u
withtype u = t * t
type t = A of int | B of u
and  u = t * t
datatype v = datatype t
type v = t = A of int | B of u
datatype complex = C of real * real
fun complex xy = C xy
fun coord (C xy) = xy




type complex = C of float * float
let complex (x,y) = C (x,y)
let coord (C (x,y)) = (x,y)
or (note parentheses in type declaration)
type complex = C of (float * float)
let complex xy = C xy
let coord (C xy) = xy

Matching

SML Ocaml
fun getOpt(NONE, d) = d
  | getOpt(SOME x, _) = x

let get_opt = function
     (None, d) -> d
   | (Some x, _) -> x
fun getOpt (opt, d) =
   case opt of
        NONE => d
      | SOME x => x
let get_opt (opt, d) =
   match opt with
        None -> d
      | Some x -> x
fun take 0 xs = []
  | take n nil = raise Empty
  | take n (x::xs) = x :: take (n-1) xs


let rec take n xs =
   match n, xs with
        0, xs -> []
      | n, [] -> failwith "take"
      | n, x::xs -> x :: take (n-1) xs
Does not have guards – use if let rec fac = function
     0 -> 1
   | n when n>0 -> n * fac (n-1)
   | _ -> raise Hell
fun foo(p as (x,y)) = (x,p,y)
let foo ((x,y) as p) = (x,p,y)

Tuples

SML Ocaml
type foo = int * float * string
type foo = int * float * string
val bar = (0, 3.14, "hi")


let bar = 0, 3.14, "hi"
or
let bar = (0, 3.14, "hi")
#2 bar



Does not have tuple selection – use pattern matching instead, e.g.
let _,x,_ = bar in x
#2



Does not have first-class selectors – use function instead, e.g.
function _,x,_ -> x
or
fun (_,x,_) -> x
(inputLine stdIn, inputLine stdIn)


Evaluation order is undefined for tuples – use let, e.g.
let line1 = read_line () in
let line2 = read_line () in
(line1, line2)

Records

SML Ocaml
type foo = {x:int, y:float, s:string ref}
Note: record types need not be declared
type foo = {x:int; y:float; mutable s:string}
Note: mutable field does not have the same type as a reference
val bar = {x=0, y=3.14, s=ref ""}
let bar = {x=0; y=3.14; s=""}
#x bar
#y bar
!(#s bar)
bar.x
bar.y
bar.s
#x

Does not have first-class selectors – use function instead, e.g.
fun r -> r.x
val {x=x, y=y, s=s} = bar
val {y=y, ...} = bar
or
val {x, y, s} = bar
val {y, ...} = bar
let {x=x; y=y; s=s} = bar
let {y=y} = bar
or (since Ocaml 3.12)
let {x; y; s} = bar
let {y; _} = bar
{x = 1, y = #y bar, s = #s bar}


{x = 1; y = bar.y; s = bar.s}
or
{bar with x = 1}
#s bar := "something"
bar.s <- "something"
Does not have polymorphic fields type bar = {f:'a.'a -> int}
{a = inputLine stdIn, b = inputLine stdIn}


Evaluation order is undefined for records – use let, e.g.
let line1 = read_line () in
let line2 = read_line () in
{a = line1; b = line2}

References

SML Ocaml
val r = ref 0
let r = ref 0
!r


!r
or
r.contents
r := 1


r := 1
or
r.contents <- 1
fun f(ref x) = x
let f {contents=x} = x
r1 = r2
r1 <> r2
r1 == r2
r1 != r2

Comparisons

SML Ocaml
2 = 2
2 <> 3
2 = 2
2 <> 3
val r = ref 2
r = r
r <> ref 2
let r = ref 2
r == r
r != ref 2
(2, r) = (2, r)
(2, r) <> (2, ref 2)
Does not have a proper generic equality
(on one hand (2, r) != (2, r), on the other (2, r) = (2, ref 2))
case String.compare(x,y) of
     LESS => a
   | EQUAL => b
   | GREATER => c
match compare x y with
     n when n < 0 -> a
   | 0 -> b
   | _ -> c
fun f x y = (x = y)
val f : ''a -> ''a -> bool


let f x y = (x = y)
val f : 'a -> 'a -> bool
Does not have equality type variables – comparison allowed on all types but may raise Invalid_argument exception
eqtype t


type t
Does not have equality types – comparison allowed on all types but may raise Invalid_argument exception

Lists

SML Ocaml
[1, 2, 3]
[1; 2; 3]
[(1, 2), (3, 4)]
[1, 2; 3, 4]
List.length xs
List.length xs
List.map f xs
List.map f xs
List.app f xs
List.iter f xs
List.foldl op+ 0 xs
List.foldr op- 100 xs
List.fold_left (+) 0 xs
List.fold_right (-) xs 100
List.all (fn x => x=0) xs
List.exists (fn x => x>0) xs
List.for_all (fun x -> x=0) xs
List.exists (fun x -> x>0) xs
val xys = ListPair.zip (xs, ys)
let xys = List.combine xs ys
val (xs, ys) = ListPair.unzip xys
let (xs, ys) = List.split xys
ListPair.app f (xs, ys)
List.iter2 f xs ys
[inputLine stdIn, inputLine stdIn]


Evaluation order is undefined for lists – use let, e.g.
let line1 = read_line () in
let line2 = read_line () in
[line1; line2]

Strings

SML Ocaml
"Hello " ^ "world\n"
"Hello " ^ "world\n"
Int.toString 13
Real.toString 3.141
string_of_int 13
string_of_float 3.141
String.size s
String.length s
String.substring(s, 1, 2)
String.sub s 1 2
String.sub(s, 0)


String.get s 0
or
s.[0]
Strings are immutable, use CharArray for mutability String.set s 0 'F'
or
s.[0] <- 'F'

Array Functions

SML Ocaml
Array.array(20, 1.0)
Array.make 20 1.0
Array.fromList xs
Array.from_list xs
Array.tabulate(30, fn x => x*x)
Array.init 30 (fun x -> x*x)
Array.sub(a, 2)


Array.get a 2
or
a.(2)
Array.update(a, 2, x)


Array.set a 2 x
or
a.(2) <- x
Array.copy{src=a, si=10, dst=b, di=0, len=20}
Array.blit ~src:a ~src_pos:10 ~dst:b ~dst_pos:0 ~len:20

Input/Output

SML Ocaml
fun copyFile(name1, name2) =
   let
      val file1 = TextIO.openIn name1
      val s     = TextIO.inputAll file1
      val _     = TextIO.closeIn file1
      val file2 = TextIO.openOut name2
   in
      TextIO.output(file2, s);
      TextIO.closeOut file2
   end
let copy_file name1 name2 =
   let file1 = open_in name1 in
   let size = in_channel_length file1 in
   let buf = String.create size in
   really_input file1 buf 0 size;
   close_in file1;
   let file2 = open_out name2 in
   output_string file2 buf;
   close_out file2
Caveat: above code actually contains a race condition.

Exceptions

SML Ocaml
exception Hell
exception Hell
exception TotalFailure of string
exception Total_failure of string
raise TotalFailure "Unknown code"
raise (Total_failure "Unknown code")
expr handle TotalFailure s =>
   ouch()
try expr with
   Total_failure s -> ouch ()

Local Declarations

SML Ocaml
fun pyt(x,y) =
   let
      val xx = x * x
      val yy = y * y
   in
      Math.sqrt(xx + yy)
   end
let pyt x y =
   let xx = x *. x in
   let yy = y *. y in
   sqrt (xx +. yy)



local
   fun sqr x = x * x
in
   fun pyt(x,y) = Math.sqrt(sqr x + sqr y)
end
Does not have local – use global declarations, an auxiliary module, or let
let
   structure X = F(A)
in
   X.value + 10
end
Standard does not have structure declarations in let but some implementations support them
let module X = F (A) in
   X.value + 10



Experimental language extension

let open M in expr end

let open M in expr
Note: since Ocaml 3.12
let
   datatype t = A | B
   exception E
in
   expr
end
Does not have local type or exception declarations – use global declarations or let module

Structures

SML Ocaml
structure X :> S =
struct
   type t = int
   val x = 0
end
module X : S =
struct
   type t = int
   let x = 0
end
X :> S
(X : S)
X : S
Does not have transparent signature ascription – use opaque ascription and with constraints
open X
include X
local open X in
   (* ... *)
end
open X
(* ... *)

Functors

SML Ocaml
functor F(X : S) =
struct
   (* ... *)
end





module F (X : S) =
struct
   (* ... *)
end
or
module F = functor (X : S) ->
struct
   (* ... *)
end
functor F(X : sig type t end) = body
structure X = F (struct type t = int end)
or
functor F(type t) = body
structure X = F(type t = int)
module F (X : sig type t end) = body
module X = F(struct type t = int end)



functor F (X : S) (Y : T) = body
Standard does not have higher-order functors but several implementations support them
module F (X : S) (Y : T) = body
or
module F = functor (X : S) -> functor (Y : T) -> body
functor F(X : S) =
let
   structure Y = G(X)
in
   Y.A
end
Does not have let for modules

Signatures

SML Ocaml
signature S =
sig
   type t
   eqtype u
   val x : t
   structure M : T
end
module type S =
sig
   type t
   type u
   val x : t
   module M : T
end
functor F(X : S) : S
Standard does not have higher-order functors but several implementations support them
module F (X : S) : S
or
module F : functor (X : S) -> S
include S
include S
Does not have open in signatures open X
structure X : A
structure Y : B
sharing type X.t = Y.u
Does not have sharing constraints – use with
S where type t = int
S with type t = int
S where X = A.B
Standard does not have where for structures but several implementations support it – use where type otherwise
S with X = A.B


signature S =
sig
   signature A
   signature B = A
end
Standard does not have nested signatures but some implementations support them
module type S =
sig
   module type A
   module type B = A
end





Original version by Jens Olsson, jenso@csd.uu.se.
Module stuff, other additions, and HTMLification by Andreas Rossberg, rossberg@mpi-sws.org.
Last modified: 2011/01/18
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