Optional output for debugging

[jmosk]

To ensure operations are executing as expected, I place several ⎕←'Value x is',x through a program. I would like to leave all of them in place to assist others in examining the operation of the code if they wish. So I would like to set up a variable 'debug' which is set to 1 to produce debug output and set to 0 to run without all the print statements.

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          debug←1  ⍝ (or 0) to turn it ON or OFF
          <conditional on debug> ⎕←'Value x is',x


I am sure others have implemented such conditional debug statements. How might have this been done?

[Morten|Dyalog]

In the last project I started, I wrote a function Log which you can see below. It allows me to decide which type of log messages I actually want to see, depending on whether I am debugging or running the application:

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mode Log msg
⍝ Log modes: (set LOGMODES to select messages to output)
⍝ D: Debug
⍝ E: Error
⍝ W: Warning
⍝ T: Transmit on WebSocket
⍝ R: Receive on WebSocket
⍝ C: Connect or Disconnect
⍝ U: Unsupported feature

→(mode∊LOGMODES)↓0
⎕←((⊃'hh:mm:ss.ffff'(1200⌶)1 ⎕DT'J'),' ',mode,':') msg


Now, you can write:

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      LOGMODES←'D'
      'D' Log 'Value x is',x
07:06:53.214 D:  Value x is 42

And when your application becomes something serious, you can start writing the messages to a proper logging mechanism.

[jmosk]

Thank you, Morten. I couldn't figure out how to eliminate the output if I was not in debug mode and never thought to abort the log function early via the →0 to avoid the output statement completely. And not realizing that →⍬ is a NOP. So many APL tricks, so little time to learn them all.

[Morten|Dyalog]

The branch to zero is an old habit, an :If statement would be nicer...

[jmosk]

Yes, but you already taught me you can't do →0 in a dfns.

[petermsiegel]

A fun way to do it in a dfn...

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  Note←{                                                             
      ⍝ ¨bool Note text¨                                             
      ⍝ bool defaults to 1                                           
      ⍝ - Use in guard of dfn (to left of colon)                     
      ⍺←1 ⋄ ⍺:0⊣⎕←⍵ ⋄ 0                                             
  }                                                                 
  MyFn←{                                                             
      ok←⍵≡⍥,1                                                       
      ⎕←'Started MyFn'                                               
      ok Note'⍵ is 1':                    ⍝ Note displays if ok is 1
      (~ok)Note'⍵ ERROR: ⍵ should be 1':  ⍝ Note displays if ok is 0
      ⎕←'Ending  MyFn'                                               
  }                                                                 
      MyFn 1
Started MyFn
⍵ is 1
Ending  MyFn
      MyFn 0
Started MyFn
⍵ ERROR: ⍵ should be 1
Ending  MyFn

Exercise left to reader.

[Adam|Dyalog]

If you use ⍞← instead of ⎕ then you can simply compress the output away:

      x←42
      debug←0
      ⍞←debug/'Value x is',x,⎕UCS 10
      debug←1
      ⍞←debug/'Value x is',x,⎕UCS 10

If you do this often, then consider defining a helper function:

      Val←{debug/'Value ',⍵,', is',(⍎⍵),⎕UCS 10}
      debug←0
      ⍞←Val'x'
      debug←1
      ⍞←Val'x'
Value x, is 42

Note how I leave out ⍞← from Val so it can be used identically in dfns and tradfns. If you want to include it, then you have to use a technique from Roger Hui's classic "assert" function (and use a trailing ":" in a dfn as per petermsiegel):

      Val←{⍞←debug/'Value ',⍵,', is',(⍎⍵),⎕UCS 10 ⋄ _←0}
      ∇ foo x
[1]     Val'x'
[2]   ∇
      debug←0
      foo 42
      debug←1
      foo 42
Value x, is 42 
      goo←{
        Val'x':
        'done'
      }
      debug←0
      foo 42 
      debug←1
      foo 42 
Value x, is 42