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

From: Louis de Forcrand
Subject: [Bug-apl] A couple of bugs, and a question on the power operator
Date: Sat, 13 Aug 2016 02:24:07 -0400

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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