Re: [Caml-list] Haskell features in O'Caml

[Florian Hars <florian@hars.de>]

Arturo Borquez schrieb am Sat, Sep 22, 2001 at 07:56:36AM -0700:
> On Sat, 22 September 2001, Steven Murdoch wrote:
> > The main one I would like is the type assertion facility of Haskell.
> > For example, one might write:
> > mul :: Int -> Int -> Int
> > mul a b = a * b
>
> As Ocaml is strong typed function mul is resolved
> to be an integer function (denoted by operator * only
> valid to integers).

Haskel is strongly typed, too, and does the same type checking.
The question was about type annotations, not type inference.

The correct answer might have been a reference to the module system.
You can specify the type of a function either in a separate *.mli-file
or in an explicit module declaration:

# module M : sig                 
    val mul : int -> int -> int  
  end = struct                   
    let mul a b = a * b          
  end;;                          
module M : sig val mul : int -> int -> int end
# M.mul 2 3;;
- : int = 6
# module N : sig               
    val mul : int -> int -> int
  end = struct                 
    let mul a b = a *. b (* Arithmetic is monomorphic - no type classes *)
  end;;                        
Signature mismatch:
Modules do not match:
[...]

The other question was about infix notation:

> > For example if max gives the maximum of two arguments
> > it can be applied normally, i.e. "max 2 3" will return 3, but "2
> > `max` 3" will also return 3.

This is missing in Ocaml (some regard this as an asset, don't ask me
why).

> > Also if a function is named using operator symbols it can be used as
> > an operator, e.g if &&& is defined as:
> Ocaml:
> # let ( &&& ) x y = if x >= y then x else y;;

But you cannot change the fixity, it is defined by the first char of
the operator, ie. ++, +*/- and +@%&!- all have the same precedence and
associativity as +.

Of course, these are syntax issues, and you can change the syntax using
camlp4:

# EXTEND                                                                
    expr: AFTER "apply"                                                 
    [[ f=expr; "{"; "["; g=expr; "]"; "}"; h=expr
       -> <:expr< $g$ $f$ $h$ >> ]];
  END;;
- : unit = ()  
# let mul a b = a * b;;
val mul : int -> int -> int = <fun>
# 3 {[mul]} 4;;
- : int = 12
# 3 {[fun a b -> a * a + 2 * a * b + b * b]} 4;;
- : int = 49

> > This can lead to some very easy to read programs

Yes. 

And messing around with delimeters may introduce subtle bugs elsewhere...
[[f=expr; "<|"; g=LIDENT; "|>"; h=expr -> <:expr< $lid:g$ $f$ $h$ >> ]];
might be better if you do not want to write applications of anonymous
functions in infix notation.

Yours, Florian.
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