[Caml-list] operators: compose and Haskell's $ operator

[Caml-list] operators: compose and Haskell's $ operator

[Charles Martin]

Does someone out there have an out-of-the box solution for adding composition and an equivalent for Haskell's $ operator to OCaml?

I've been using let (<<) f g x = f (g x) for composition, but an equivalent for the $ operator eludes me, as it needs to be right associative and of low precedence.  Maybe someone with some Haskell envy has figured something out using camlp4?

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Re: [Caml-list] operators: compose and Haskell's $ operator

[Brian Rogoff]

On Wed, 25 Apr 2001, Charles Martin wrote:
> Does someone out there have an out-of-the box solution for adding
> composition and an equivalent for Haskell's $ operator to OCaml?
> 
> I've been using let (<<) f g x = f (g x) for composition, but an

You should be warned that that doesn't mix nicely with CamlP4's quotation 
syntax.

>equivalent for the $ operator eludes me, as it needs to be right
>associative and of low precedence.

Maybe <->? Let's test it

# let ( <-> ) f x = f x;;
val ( <-> ) : ('a -> 'b) -> 'a -> 'b = <fun>
# (fun x -> x + 1) <-> (5 / 2) + 3;;
- : int = 6
# (fun x -> x + 1) <-> (5 / 2) * 3;;
- : int = 7

> Maybe someone with some Haskell envy has figured something out using
> camlp4?

I admit I have a little Haskell envy (type classes and the `infix` syntax :), 
but I can probably live with some parentheses ...

-- Brian


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Re: [Caml-list] operators: compose and Haskell's $ operator

[Charles Martin]

At 05:57 PM 4/25/01 -0700, you wrote:
>On Wed, 25 Apr 2001, Charles Martin wrote:
>>equivalent for the $ operator eludes me, as it needs to be right
>>associative and of low precedence.
>
># let ( <-> ) f x = f x;;
>val ( <-> ) : ('a -> 'b) -> 'a -> 'b = <fun>

Unfortunately, this is left-associative:

# (+) 3 <-> ( * ) 2 <-> 4;;
Characters 0-5:
This expression has type int -> int but is here used with type
  (int -> int) -> 'a

...and higher in precedence than logical operators:

# not <-> true && false;;
- : bool = false

The expression table (http://caml.inria.fr/ocaml/htmlman/manual014.html) does not have an ellipsis after the "<-" entry, so I assume that "<->" gets lumped into the "other infix symbols" category, which is left associative (and higher precedence than the logical operators).


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Re: [Caml-list] operators: compose and Haskell's $ operator

[Brian Rogoff]

On Wed, 25 Apr 2001, Charles Martin wrote:
> At 05:57 PM 4/25/01 -0700, you wrote:
> >On Wed, 25 Apr 2001, Charles Martin wrote:
> >>equivalent for the $ operator eludes me, as it needs to be right
> >>associative and of low precedence.
> >
> ># let ( <-> ) f x = f x;;
> >val ( <-> ) : ('a -> 'b) -> 'a -> 'b = <fun>
> 
> Unfortunately, this is left-associative:

Oops, you're right of course. Given that there seems to be no solution, I
guess a little extra parenthesization is your only "pure OCaml option" option. 

I've only seen the $ used heavily in code that uses the do notation, where 
OCaml might use (gasp!) imperative code with ;, so I've never been tempted 
to write a syntax extension for this. 

-- Brian


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