[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Defining new Scheme predicates
From: |
Urs Liska |
Subject: |
Re: Defining new Scheme predicates |
Date: |
Mon, 7 Nov 2016 11:07:45 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 |
Am 07.11.2016 um 11:03 schrieb Urs Liska:
>
> Am 07.11.2016 um 09:57 schrieb Richard Shann:
>> On Mon, 2016-11-07 at 06:45 +0100, Urs Liska wrote:
>>> Am 7. November 2016 01:20:23 MEZ, schrieb Andrew Bernard <address@hidden>:
>>>> Hi Simon,
>>>>
>>>> Thanks! Exactly perfect. Sometimes the completely obvious escapes me.
>>>> Better
>>>> have another coffee.
>>>>
>>>> Most appreciated.
>>>>
>>>> I suppose of course that to make it a predicate without the preliminary
>>>> let
>>>> block (not that I have any objection to that) one would have to modify
>>>> lilypond internals, which would not be desirable.
>>> Not at all!
>>>
>>> Just define your predicate with
>>>
>>> #(define (side? obj)
>>> (if (or (eq? obj 'left)
>>> (eq? obj 'right))
>>> #t #f))
>> more succinctly
>>
>> #(define (side? obj)
>> (or (eq? obj 'left)
>> (eq? obj 'right)))
> No. This will return either 'left or 'right if successful. But a
> predicate is to return #t or #f.
> Urs
Oops, forget about that. I mixed that up with another case. eq? *will*
return #t or #f.
>
>>> and use it like any other procedure. The ? at the end is just a convention,
>>> predicates are nothing else
>>> than procedures taking one argument and returning #t or #f.
>> #t or any other value that is not #f - the value #t is rather rarely
>> used in conventional Scheme code; great use is made of the convenience
>> that all expressions are true except #f which is false.
But here my previouso comment applies. Predicates should return #t or #f.
Of course there are many valid and valuable use cases of the "non-false
expression" appraoch.
See also
https://scheme-book.ursliska.de/scheme/procedures/predicates.html and
https://scheme-book.ursliska.de/scheme/conditionals/if.html
Urs
>> Richard
>>
>>
>>
>>
>>
>>
>
> _______________________________________________
> lilypond-user mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/lilypond-user
- Defining new Scheme predicates, Andrew Bernard, 2016/11/06
- Re: Defining new Scheme predicates, Simon Albrecht, 2016/11/06
- Re: Defining new Scheme predicates, Simon Albrecht, 2016/11/06
- RE: Defining new Scheme predicates, Andrew Bernard, 2016/11/06
- Re: Defining new Scheme predicates, Thomas Morley, 2016/11/06
- RE: Defining new Scheme predicates, Urs Liska, 2016/11/07
- Re: Defining new Scheme predicates, Richard Shann, 2016/11/07
- Re: Defining new Scheme predicates, Urs Liska, 2016/11/07
- Re: Defining new Scheme predicates,
Urs Liska <=
- Re: Defining new Scheme predicates, David Kastrup, 2016/11/07
- Re: Defining new Scheme predicates, David Kastrup, 2016/11/07