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

[Louis de Forcrand]

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