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please add List.cons #4714

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vicuna opened this issue Feb 11, 2009 · 6 comments

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commented Feb 11, 2009

Original bug ID: 4714
Reporter: ecc
Assigned to: @gasche
Status: closed (set by @xavierleroy on 2016-12-07T10:48:59Z)
Resolution: fixed
Priority: normal
Severity: feature
Version: 3.11.0
Fixed in version: 4.03.0+dev / +beta1
Category: ~DO NOT USE (was: OCaml general)
Monitored by: @dbuenzli

Bug description

Since :: is treated specially, one cannot use ( :: ) as a shorthand for the function (fun x y -> x :: y). It would be convenient to define List.cons to be this function, complementing List.hd and List.tl.

(Unless this would be offensive to les francophones, in which case another name would be fine :-)

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commented Feb 12, 2009

Comment author: @dbuenzli

I also needed this function really more than once. Daniel

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commented Mar 16, 2009

Comment author: @damiendoligez

First, :: is a constructor and in general in OCaml, you cannot use a constructor name as a shorthand for one the corresponding construction functions.
Second, :: only looks special because it is an infix symbol, but like any other infix symbol, if you enclose it in parentheses it is parsed as a prefix symbol.

For example: let x = (::) (1, []);;

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commented Mar 20, 2009

Comment author: @dbuenzli

Yes but what we need is to use as a function and you cannot do that. See the following toplevel session :

    Objective Caml version 3.11.0

( < );;

  • : 'a -> 'a -> bool =

( :: );;

Error: Syntax error

(::);;

Error: Syntax error

Thus the need for List.cons e.g. to pass to folding functions.

Best,

Daniel

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commented Mar 20, 2009

Comment author: @damiendoligez

You cannot do that for any constructor. Why is there a particular need for :: ?
Since the function definition is only a few characters, it's not clear that we'll
want to "pollute" the stdlib namespace in this case. We'll have to argue it
between the OCaml developers to make a decision.

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commented Mar 20, 2009

Comment author: @dbuenzli

While it is at first suprising that (::) cannot be used as a function as the other operators allow, I don't really care about this. But having List.cons instead of (fun a b -> a :: b) could be nice and more readable in certain cases; I know I missed it a few times.

OTOH, for (left) folds List.cons wouldn't be in the right order anyway.

Best,

Daniel

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commented Apr 26, 2015

Comment author: @gasche

List.cons should be included in 4.03.

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