Re: [Caml-list] Examples where let rec is undesirable

[Gabriel Scherer <gabriel.scherer@gmail.com>]

> What I am wondering is why type definitions are recursive by default. It
> is mostly troubling for beginners.

Indeed, it is a defect of the language not to have non-recursive type
definitions. My usual reference on this topic is Yaron's complaint:
  https://ocaml.janestreet.com/?q=node/25

The problem is not so much that they are "recursive by default" (I
believe this would be justified by the relative frequencies of
recursive type definitions and type-level name recycling) but that
they *must* be recursive. It would be better to have some kind of
"type-nonrec" construct for non-recursive type declarations.

This is however a case were it is difficult to fix the issue in a
fully convenient way: you don't want to add a new keyword, you
certainly don't want to change the behavior of existing "type"
declarations, and all solutions remaining are likely to be convoluted
and feel like a ad-hoc patch.
Given there is the reasonable (if ugly) workaround of breaking the
cycle through an auxiliary type renaming, there are few incentives to
change the state of the art. I believe this is something you could and
should easily fix when designing a new language (when introducing
*any* binding construct, think about both a recursive (when it makes
sense) and a non-recursive version), but that may have to stay with us
for OCaml.

On Tue, Jan 3, 2012 at 6:47 AM, Martin Jambon
<martin.jambon@ens-lyon.org> wrote:
> On 01/02/2012 04:05 PM, Lukasz Stafiniak wrote:
>> On Mon, Jan 2, 2012 at 11:37 PM, Diego Olivier Fernandez Pons
>> <dofp.ocaml@gmail.com> wrote:
>>>     List,
>>>
>>> I was wondering if there was any reason not to make "let rec" the default /
>>> sole option, meaning cases where you clearly don't want a "let rec" instead
>>> of "let" (only in functions, not cyclic data).
>>>
>>>          Diego Olivier
>>
>> The default "no-rec" allows for name recycling -- using the same name
>> for an incrementally transformed value, i.e. to bind the intermediate
>> results. Name recycling minimizes the cognitive burden: there are less
>> names to remember in a scope, and differences in names are justified
>> by differences in purpose of the values. Are there reasons to consider
>> name recycling a bad style?
>
> What I am wondering is why type definitions are recursive by default. It
> is mostly troubling for beginners.
>
> The following is an occasional inconvenience of having only recursive
> type definitions:
>
> module A =
> struct
>  type t = ...
>  let compare = ...
>
>  module B =
>  struct
>    type t = t (* uh oh *)
>    let compare = compare
>  end
> end
>
>
> Martin
>
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