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 |0|s ?
Sincerely,
-- Emmanuel Charpentier
