List.cons and "::"

List.cons and "::"

[Sam Steingold]

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Why is :: a syntax, not an infix version of List.cons
(which for some reason is missing)?
thanks.

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Re: [Caml-list] List.cons and "::"

[Brian Hurt]

Sam Steingold wrote:

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>
>Why is :: a syntax, not an infix version of List.cons
>(which for some reason is missing)?
>thanks.
>  
>

So it can be used in pattern matching.  Things used in pattern matching 
the compiler has to know about, and thus the user can't override.  
Things not used in pattern matching can be overridden.  So you can 
redefine + if you wanted to.  I wouldn't recommend it, but you could.  
:: and , you can't override.

Brian

Re: [Caml-list] List.cons and "::"

[Martin Jambon]

On Tue, 17 Jul 2007, Brian Hurt wrote:

> Sam Steingold wrote:
>
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA1
>> 
>> Why is :: a syntax, not an infix version of List.cons
>> (which for some reason is missing)?
>> thanks.
>> 
>> 
>
> So it can be used in pattern matching.  Things used in pattern matching the 
> compiler has to know about, and thus the user can't override.  Things not 
> used in pattern matching can be overridden.  So you can redefine + if you 
> wanted to.  I wouldn't recommend it, but you could. 
> : :  and , you can't override.

:: and () are just constructors with a special syntax.
You can override them:

         Objective Caml version 3.09.3

# type zarbi = :: of int * int | ();;
type zarbi = :: of int * int | ()
# ();;
- : zarbi = ()
# 1 :: 2;;
- : zarbi = :: (1, 2)


It's just that constructors are not automatically associated with a 
function, in OCaml.


Martin

--
http://martin.jambon.free.fr

Re: [Caml-list] List.cons and "::"

[Chris King]

On 7/17/07, Brian Hurt <bhurt@janestcapital.com> wrote:
> I wouldn't recommend it, but you could.
> :: and , you can't override.

Actually,

# type foo = :: of int * int;;
# 1 :: 2;;
- : foo = :: (1, 2)

:)  This is because :: is considered a constructor, and not just
syntax (as is , or []).

But in general automatic currying of constructors would be nice (I
think Haskell does this).  There is probably some technical reason why
it's not a good idea in O'Caml though.

- Chris

Re: List.cons and "::"

[code17]

Martin Jambon <martin.jambon@ens-lyon.org> writes:
> :: and () are just constructors with a special syntax.
... and so do "true" and "false".  

> You can override them:
... and the way of pattern matching (::) remains the same. 

[cf.] http://ocaml.cn/node/344

Re: [Caml-list] List.cons and "::"

[Jon Harrop]

On Tuesday 17 July 2007 21:02:29 Sam Steingold wrote:
> Why is :: a syntax, not an infix version of List.cons

List.cons would be a function whereas :: is a type constructor for the 'a list 
type:

# type 'a list = :: of 'a * 'a list;;
type 'a list = :: of 'a * 'a list
# let rec list = 1 :: list;;
val list : int list =
  :: (1, ...

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
OCaml for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

Re: [Caml-list] List.cons and "::"

[Pierre Etchemaïté]

Le Tue, 17 Jul 2007 16:46:34 -0400, "Chris King" <colanderman@gmail.com> a écrit :

> But in general automatic currying of constructors would be nice (I
> think Haskell does this).  There is probably some technical reason why
> it's not a good idea in O'Caml though.

http://caml.inria.fr/pub/ml-archives/caml-list/2001/08/47db53a4b42529708647c9e81183598b.en.html