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

[Juergen Sauermann]

Hi,

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?

Regards,
Elias

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

solutions:

- 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 ⍬

 TCTGAT  ATCTGAT 

 TGCATA  GCATA   

 ATCTGAT TGCATA  

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

 TCTGAT  ATCTGAT 

 TGCATA  GCATA   

 ATCTGAT TGCATA

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

three column matrix.

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

      )SI

⋆⋆  

==============================================================================

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,

Louis