I'm sorry, _I_ misunderstood the manual. I did not know that Dyalog dfns were
always dyadic / ambivalent.
If I understand correctly then, in F {pow} G, G's left argument is _always_ the
result of the power operator if G is true. Its right argument then is _always_
the result of the previous iteration. Is that correct?
If it is, I believe always ambivalent lambdas would be convenient (aside from
possible implementation problems), but not at all essential; it's probably rare
to check something about the previous iteration without comparing it to the
current one. But wouldn't have been simpler to model them as ambivalent from
the ground up?
Louis
> On 15 Aug 2016, at 08:22, Jay Foad <[email protected]> wrote:
>
> The manual is here, see page 145:
> http://docs.dyalog.com/15.0/Dyalog%20APL%20Language%20Reference%20Guide.pdf
>
> I'm not sure where Louis got the information about monadic G. I assure you
> that Dyalog APL does not examine G to see if it's monadic or dyadic. It
> always tries to apply G dyadically.
>
> Jay.
>
>> On 15 August 2016 at 12:30, Juergen Sauermann
>> <[email protected]> wrote:
>> Hi Jay,
>>
>> I see. Maybe I misunderstood Louis,s email from last Saturday completely?
>> The way I read this email was that in Dyalog APL version 15 you can have
>> a monadic condition function G in F⍣G . Quote from the email:
>>
>>
>> The Dyalog 15.0 manual states that the power operator can take a
>> function right argument. In this case, that function can be
>> either monadic or dyadic, and can be a lambda.
>> If it’s monadic:
>>
>> (F⍣G) ⍵ ←→ ⍵ ← F ⍵ until G ⍵
>> ⍺ (F⍣G) ⍵ ←→ ⍵ ← ⍺ F ⍵ until G ⍵
>>
>> If it’s dyadic:
>>
>> (F⍣G) ⍵ ←→ ⍵ ← F ⍵ until ( F ⍵) G ⍵
>> ⍺ (F⍣G) ⍵ ←→ ⍵ ← ⍺ F ⍵ until (⍺ F ⍵) G ⍵
>>
>> (Note that G is checked before the first time F is executed.)
>>
>> I don't know if that statement is correct or not, but if it is then I would
>> prefer to not
>> introduce this "monadic case" in GNU APL for the reasons explained earlier.
>>
>> Thanks,
>> Jürgen
>>
>>
>>> On 08/15/2016 10:16 AM, Jay Foad wrote:
>>>> On 13 August 2016 at 13:05, Juergen Sauermann
>>>> <[email protected]> wrote:
>>>> In "Mastering Dyalog APL" I haven't found the monadic case for the right
>>>> function argument
>>>> G of the power operator. In that book G seems to be always dyadic. So the
>>>> monadic case looks
>>>> like a new Dyalog invention. And, if it is defined like you say, IMHO not
>>>> the ultimate wisdom.
>>>
>>> There is no "monadic case". G is always applied dyadically, and if it
>>> happens to be a strictly monadic function then you'll get a SYNTAX ERROR:
>>>
>>> ⎕FX'r←g y' 'r←g>10' ⍝ g is strictly monadic
>>> 2 g 4 ⍝ applying it dyadically gives an error
>>> SYNTAX ERROR
>>> (+⍨⍣g)1 ⍝ power operator tries to apply g dyadically
>>> SYNTAX ERROR
>>>
>>> In general, Dyalog APL never examines a function operand to see whether it
>>> is monadic or dyadic, in order to treat them differently.
>>>
>>> Jay.
>
