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[Bug-apl] Fwd: A couple of bugs, and a question on the power operator

From: Louis de Forcrand
Subject: [Bug-apl] Fwd: A couple of bugs, and a question on the power operator
Date: Sun, 14 Aug 2016 01:40:59 -0400

And a third email about lambda locals.

IMHO, a solution could be to introduce an extra assignment arrow which denotes explicit
global assignment, and make the usual assignment arrow denote assignment which is
local to lambdas (or have it the other way around).
If the standard doesn’t allow something like this, maybe something along the lines of ⎕FX

‘A’ ⎕LC data
‘A’ ⎕GL data

to denote explicit global or local assignment useful in lambdas or other places, whichever
is not chosen to be denoted by the already existing arrow, could be used.
I would argue that the current arrow be used for local assignment, because (aside from
personal taste) global assignment is more “dangerous” if inadvertently used. As in,
a person who wants to just throw together a quick lambda doesn’t usually want
or need the effects of global assignment unless they can explicitly say so.


Sorry if I’m sending these twice; my mailer automatically sends them only to you Jürgen,
so I end up forwarding them to the mailing list.

On 13 Aug 2016, at 10:19, Juergen Sauermann <address@hidden> wrote:


the problem is that there is no syntactical means to "state otherwise".

I personally find it convenient to assign to variables outside the lambda, for example
to increment a counter on every iteration of the lambda. Of course one can criticize the way
APL handles variables, but the limited number of function arguments and return values
makes the scoping rules of APL almost inevitable. And I still believe that it is a bad thing
if the scoping rules of lambdas are different from the scoping rules of "normal" defined functions.

So the only room for changes that remains is to have a way of adding local variables to the header
line of lambdas. Something along the lines of:

      FUN←'A B C' ⎕LOCALIZE { ... }

where A B and C would become local variables in the named lambda FUN.

I would also argue that lambdas are only a quick-and-dirty hack for specifying the function arguments
of the EACH operator and friends; for serious functions ⎕FX and ∇ have all the features that are missing
in GNU APL lambdas.

/// Jürgen

On 08/13/2016 02:11 PM, Elias Mårtenson wrote:

With regards to lack of local variables for lambda functions, I've observed that local is the typical case, and nonlocal is somewhat rare.
Wouldn't it make sense to make all variable assignments local for lambda functions local, unless stated otherwise?

On 13 Aug 2016 8:05 pm, "Juergen Sauermann" <address@hidden> wrote:
Hi Louis,

a quick answer to your question beforehand, I will look into the bugs later.

GNU APL has implemented the power operator according to the description
in the book "Mastering Dyalog APL". The ISO standard says nothing about this
operator, it is simply not defined there.

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.

What I do not like at all is the fact that in the dyadic case F is being computed before G (it has to,
because you need the result ⍺F⍵ or F⍵ as left argument of G. And, as you say, in the monadic
case F is being computed after G. 

I could imagine to implement the monadic G if it were always computed after F so that the order
of execution is consistent for the monadic and dyadic cases. But if Dyalog really has different orders
then I would prefer to not implement the monadic case in GNU APL at all because then I would only
have the choice between an inconsistent execution order (which is, in my opinion, bad) or an
incompatibility with Dyalog APL (which is also bad).

Actually in GNU APL lambdas can be ambivalent, but monadic functions (lambda or not) cannot be
ambivalent. Localizing variables assigned in lambdas automatically is technically possible but would
break backward  compatibility of existing workspaces. There was a discussion on this topic earlier this
year, but no progress lately. I was thinking of some ⎕-function (like ⎕FX but with {} syntax that would
allow you to created lambdas with more control over the localization of variables).

BTW, I read the word until in your description below as F being called before G and not after G,
even in the monadic case.

/// Jürgen

On 08/13/2016 08:24 AM, Louis de Forcrand wrote:

I’ll start with the question:

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 what the ISO standard says on this, but in GNU APL,
dyadic G works as in Dyalog. However, “monadic” lambda G has to
be a weird function that takes both a left and a right argument,
and discards the left one. That is:

Dyalog: GNU:
F⍣{G ⍵} F⍣{⍺⊢G ⍵}

Is this because lambdas can’t be ambivalent? If so, I see two

- Make ⍣ check G’s valence.

- Better: I know it’s possible to write an ambivalent tradfn
  (function defined with the ∇-editor) by using ⎕NC on the
  left argument; wouldn’t it be possible to implement lambdas
  containing only ⍵ as ambivalent, so ⍺ is simply never used
  even if it is defined (or defined even if it isn’t used)?
  In fact, dyadic tradfns work in this way.

Not only would this allow for cleaner use of ⍣, but it would
also allow for “cleaner” case statements in lambdas:

{⍎(‘case0’ ‘case1’ ‘case2’ ‘etc.’)[condition]}

which is probably the only place one would use this.

As of now, if ⍺ is present in one of the case statements but
not in the rest of the function, then ⍺⊢ must be prepended to
the lambda.

While on the subject of lambdas, IMHO variables assigned inside
lambdas should be made local. More than once I’ve used a named
lambda in a tradfn and have found that one of its local variables
was modified by the lambda. Although I imagine named lambdas must
be a pain to implement.

Also, I noticed that the assignment of a lambda to a name
returns a vector of the name of the lambda. It would be
interesting if it could return the actual lambda (of course this
probably isn’t feasible, since it would require function
returning expressions, a.k.a. tacit programming).

Now on to the bugs:

      (g d)←'ATCTGAT' 'TGCATA'
      {((1↓X)Y((⊃X),Z)),[¯.5]X(1↓Y)((⊃Y),Z)⊣(X Y Z)←⍵}g d ⍬
      {((1↓X)Y((⊃X),Z)),[.5]X(1↓Y)((⊃Y),Z)⊣(X Y Z)←⍵}g d ⍬

These should be transposed. ⎕IO ←→ 0, so ,[¯.5] should give a two row,
three column matrix.

←———————            -            -            -            ———————→ 


Assertion failed: idx < items_valid
in Function:      operator[]
in file:          ./Simple_string.hh:140

Call stack:

-- Stack trace at ./Simple_string.hh:140
0xa @@@@
0xa  @@@@
0xa   @@@@
0xa    @@@@
0xa     @@@@
0xa      @@@@
0xa       @@@@
0xa        @@@@
0xa         @@@@

SI stack:

Depth:      9
Exec:       0x7f98db4177b0
Safe exec:  0
Pmode:    ⍎  (1↓R)((=/0⌷¨V)↓⍵⊃⍨~⍺)((2⊃⍵),1↑R←⍺⊃V)
PC:       27 /
Stat:     (1↓R)((=/0⌷¨V)↓⍵⊃⍨~⍺)((2⊃⍵),1↑R←⍺⊃V)
err_code: 0x50005
thrown:   at Value.cc:1051
e_msg_1:  'INDEX ERROR+'
e_msg_2:  '      (1↓R)((=/0⌷¨V)↓⍵⊃⍨∼⍺)((2⊃⍵),1↑R←⍺⊃V)'
e_msg_3:  '               ^           ^'

Depth:      8
Exec:       0x7f98db41b9f0
Safe exec:  0
Pmode:    ∇ 
Assertion failed: idx < items_valid
in Function:      operator[]
in file:          ./Simple_string.hh:140

Call stack:
*** do_Assert() called recursively ***
*** immediate_execution() caught other exception ***

I’m sorry I couldn’t include the input preceding that )SI, it's long and very
hard to reproduce. I hope this is enough to help :-⌈


I’d like to thank you for your very hard work. I don’t know of any other APL
which adheres to the standard while being as bug free as yours, and is a
one-man project. Let alone all three!
Keep in mind everything I suggest is the personal opinion of someone with very
little experience with C++ish languages. I’m sure you know better than I do how
to shape your APL. After all, you’re the one writing it!

Best of luck,

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