[ Copy of my answer to Elias, which somehown didn't make it to bug-
apl... ]
Dear list,
thank you all for your answers, which help me seriously in my discovery
of APL ... 40 years after !
--
Emmanuel Charpentier
Le vendredi 09 juin 2023 à 02:04 +0800, Elias Mårtenson a écrit :
> You note an inconsistency, but the problem isn't worth the lambda
> syntax but rather with assignment. In the lambda syntax of Dyalog,
> the left arrow are used both to assign functions as well as values,
> which makes the evaluation rules complicated.
>
> I came across the same problem when I implemented it in KAP. In my
> case it was even worse since KAP always parses from left to right and
> generated a syntax tree before evaluation, so the ad hoc person of
> Dyalog is even more ugly.
>
> I chose to use a different symbol for assigning to functions. I use ⇐
> instead of ← of this purpose. It turned out to work pretty well.
>
> Den fre 9 juni 2023 01:44Dr. Jürgen Sauermann
> <mail@jürgen-sauermann.de> skrev:
> >
> > Hi Emmanuel,
> >
> > first of all, lambdas are a bad construct which causes quite
> > a number of syntactic inconsistencies. Simply speaking,
> > the concept has not been thought out well.
> >
> > 1. A defined function created with ∇ or ⎕FX is unambiguously
> > that: a defined function. For that reason I sometimes call
> > them "proper functions" (as opposed to lambdas which are
> > anything but proper).
> >
> > 2. The stone-old fundamental APL evaluation rule reads (quote
> > from the IBM APL2 Language Reference Manual, page 20):
> >
> > EVALUATION OF EXPRESSIONS
> >
> > All functions execute according to their position within an
> > expression. The rightmost
> > function whose arguments are available is evaluated first.
> >
> > 3. Now look at, for a simple example:
> >
> > Lambda ← { ⎕TS }
> >
> > According 2. above (and noting all functions, which certainly
> > includes the
> > special case of niladic functions), the niladic { ⎕TS } is the
> > rightmost
> > function whose arguments (i.e. none since the { ... } is niladic)
> > has to be
> > computed first.As of this writing, the result is, say,
> >
> > { ⎕TS }
> > 2023 6 8 19 25 43 139
> >
> > and what remains is:
> >
> > Lambda ← 2023 6 8 19 25 43 139
> >
> > 4. IOW: what looks at the first glance like the definition of a
> > niladic function
> > named Lambda is actually the assignment of a 7-item vector to
> > variable
> > Lambda.
> >
> > 5. Even more interesting, the inventor of the {...} notation (as
> > far as i know)
> > says (on tryapl.org) the following:
> >
> > Lambda ← { ⎕TS }
> >
> > )FNS
> > Lambda
> >
> > Lambda ⍝ expecting e.g. 2023 6 8 19 25 43 139
> > {⎕TS}
> >
> > which makes, IMHO, even less sense.
> >
> > Hope this explains it,
> >
> > Jürgen
> >
> >
> >
> > On 6/8/23 17:37, Emmanuel Charpentier wrote:
> >
> > >
> > > Dear list,
> > >
> > > It seems that niladic lambdas are treated like constants.
> > >
> > > Rough and naïve illustration : pasting this :
> > >
> > > ⍝ What about niladic functions and lambdas ?
> > > ⍝ Example of a numeric timestamp generator
> > > ⍝ Simplifying assumptions : we want to measure about a few
> > minutes
> > > ⍝ and not around midnight...
> > > ⍝ Start afresh
> > > )clear
> > > ⍝ Function
> > > ∇ R ← NTS
> > > R ← 24 60 60 1000 ⊥ ¯4↑⎕TS
> > > ∇
> > > ⍝ Try it
> > > T1 ← NTS
> > > ⍝ Wait a few seconds
> > > )host sleep 2
> > > T2 ← NTS
> > > ⍞←'Spent Time : ',⍕(T2-T1)÷1000
> > > ⍝ Lambda
> > > nts ← {24 60 60 1000 ⊥ ¯4↑⎕TS}
> > > ⍝ Try it
> > > t1 ← nts
> > > ⍝ Wait a few seconds
> > > )host sleep 2
> > > t2 ← nts
> > > ⍞←'Spent time : ',⍕(t2-t1)÷1000
> > >
> > > in a gnu-apl buffer gives :
> > >
> > > CLEAR WS
> > >
> > > 0
> > > Spent Time : 2.002
> > > 0
> > > Spent time : 0
> > >
> > > Why ?
> > >
> > > Bonus question : what causes the impression of the 0s ?
> > >
> > > Sincerely,
> > >
> > > -- Emmanuel Charpentier
> > >
> >
