Selective Expression Tracing

Operator trace from dfns.dws is a simple and low-tech tool for displaying intermediate results of functions, during the evaluation of an expression. For example, what’s going on here?

      (+⌿ ÷ ≢) 1 2 3 4
2.5

We can guess that the above fork is computing the average of the right argument. To see how this works:

      )copy dfns trace
...
⍝ Rename to reduce clutter
      t ← trace
⍝ Bind each "tine" of the fork with t to see what's happening
      (+⌿t ÷t ≢t) 1 2 3 4
≢  1 2 3 4  =>  4
+⌿  1 2 3 4  =>  10
10  ÷  4  =>  2.5
2.5
⍝ But how is that +⌿ reduction evaluated?
      (+t⌿ ÷ ≢) 1 2 3 4
3  +  4  =>  7
2  +  7  =>  9
1  +  9  =>  10
2.5

As a second example, what does the atop (~⍷) do in the following function to remove superfluous '.' characters from its argument?

      {('··'(~⍷)⍵)/⍵} 'squeeze·····me'
squeeze·me
      {('··'(~⍷)t⍵)/⍵} 'squeeze·····me'
··  ~⍷  squeeze·····me  =>  1 1 1 1 1 1 1 0 0 0 0 1 1 1
squeeze·me

…and so forth. Notice how t can be injected to the right of any of the functions in an expression.
For more examples, see the notes for trace.