[erlang-questions] Infix function and user-defined operators

Richard O'Keefe <>
Wed Jan 16 15:15:52 CET 2019


For what it's worth, Fortran also has user-defined operators, of
the form /[.][[:alpha:]]+[.]/.  I fear that back in 2004 too many
people confused *user-defined* *alphabetic* operators with
*overloaded* or *symbolic* operators, or even user-defined operators
with user-selected precedence.  (And some forgot that '#!$@' is
already a perfectly good function name...)

I recall Bill Clocksin (of "Clocksin & Mellish" Prolog textbook fame)
saying that if he ever had a chance to redesign Prolog the one thing
he would improve would be to delete user-defined operators...

In a functional language I expect there to be fewer opportunities to
use prefix or infix operators than in Fortran, because you tend to
need more arguments.

Speaking of Fortran, that may be why I dislike /foo/ (looks like a
COMMON block name), and of course in most languages 1/foo/2 would be
an arithmetic expression with two divisions, which would confuse me.


On Fri, 11 Jan 2019 at 21:18, Dmitry Kolesnikov <>
wrote:

> Hello Richard,
>
> Thank you for valuable feedback.
>
> Indeed `name` is the best option. However, it requires changes to the
> compiler. Unfortunately, it do not have any visibility why this feature has
> been rejected 14 years ago. I found that /name/ is the closed approximation
> of Haskell’s `name` achievable with core Erlang syntax thus implementable
> via parse transforms.
>
> Current proposal of user defined operator is left associative with same
> precedence as multiplication.
>
> I also thought about /f> and <f/ similarly to MLton but have not build a
> final conclusion yet. The “single” character operators can be achieved via
> /$_/ with some limitation. One of the problem here is name of module to
> conduct its implementation.
>
>
> Best Regards,
> Dmitry
>
>
> On 8 Jan 2019, at 15.53, Richard O'Keefe <> wrote:
>
> The Haskell convention for user-defined operators is `name`  .
> I don't see any point in inventing a new one.
> I note that
> (a) http://mlton.org/InfixingOperators shows a relatively simple
> scheme for ML that would fit Erlang without too much strain.  It
> starts with a warning that operator properties are not part of
> module interfaces.  This was a problem in Prolog too, and I
> surmise that it may have been the reason for not having user-
> declared operators in Erlang.
> (b) In F#, the type and precedence of a user-defined operator
> are determined by its first character.  See
>
> https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/symbol-and-operator-reference/index
> (c) ECMA Eiffel 8.28.5 puts all unary user-defined operators at the
> same precedence level and all binary user-defined operators at another
> precedence level.  Some other Eiffel dialects followed the same rule as
> F#.  The (b) and (c) rules appear to also be related to not having
> operator properties in signatures/classes/modules/whatever.
>
> More years ago than I care to admit, I wrote an extended
> Prolog parser that allowed user-defined "operators" like
> "N of L are V" or "at least N of L are V".  It was based
> on Vaughan Pratt's CGOL; see
> https://en.wikipedia.org/wiki/CGOL
>
>
>
> On Tue, 8 Jan 2019 at 04:56, Dmitry Kolesnikov <>
> wrote:
>
>> Hello Community,
>>
>> Now and then, I am missing an infix notation in Erlang. There was a great
>> discussion about this subject decade and half ago but I do not think
>> anything is changed [1].
>>
>> I am testing and stretching limits of functional abstracts in Erlang as
>> part of my pure functional and generic programming exercises [2]. I’ve
>> decided to experiment with Infix function via parse-transform.
>>
>> The parse_transform feature implements a syntax sugar for infix, which is
>> compiled into valid Erlang code using corresponding function at
>> compile-time. To apply a function using infix notation, we enclose its name
>> in slash characters (/). This allows us to use any arity 2 functions as
>> infix operators.
>>
>> ```
>> -compile({parse_transform, infix}).
>>
>> 1 /plus/ 1.
>>
>> F /lists:map/ [1, 2, 3].
>> ```
>>
>> Right now, I am working on this feature here
>> https://github.com/fogfish/datum/pull/64, you can find more detailed
>> description there, code, etc.
>>
>> I would appreciate for any feedback, suggestion from the community.
>> Does it sounds any usable, still desired or total braindead?
>> What other use-case/applications you might imagine for infix notation?
>>
>> Best Regards,
>> Dmitry
>>
>> References
>>
>> 1. http://erlang.org/pipermail/erlang-questions/2004-March/011929.html
>> 2. https://github.com/fogfish/datum
>>
>>
>>
>>
>>
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>> 
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>>
>
>
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