Re: Defining new Scheme predicates

[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

>
>> 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.
>
> Richard
>
>
>
>
>
>