rslt ← (dewy rarg         ⍝ Apply no-result function "en passant".

A  function [sic] that doesn't return its result is an embarrassment to  the  D-
programmer.  Operator  [do]  applies such a function to its argument and returns
the argument as result.

    {} do 3

For example, suppose we want a D-fn "wset" which, given a ref to an object and a
property-value  pair, sets the new value and returns the previous one as result.
We might be tempted to try something along the lines of:

    wset←{                  ⍝ Set new prop-value ⍵ on object ⍺.
        rslt←⍺ ⎕WG ⎕IO⊃⍵    ⍝ get old value.
        ⍺ ⎕WS ⍵             ⍝ set new value.       !!!!!!! wrong
        rslt                ⍝ return old value.

This  won't work because the function terminates on the second line (⎕WS) before
returning  "rslt"  on  the  third. We can't use a local assignment on the second
line to prolong execution:

        _←⍺ ⎕WS ⍵           ⍝ set new value.       !!!!!!! wrong

because  ⎕WS  does  not (currently) return a result and so would produce a VALUE
ERROR. So we use [do] in the following way. Note that, as [do] derives a monadic
function  (takes no left argument), we are obliged to bind ⎕WS's left argument ⍺
to form a monadic operand ⍺∘⎕WS:

        _←⍺∘⎕WS do ⍵        ⍝ set new value.
        0⊣⍺∘⎕WS do ⍵:       ⍝ set new value.

or we could combine the final two lines into:

    wset←{                  ⍝ Set new prop-value ⍵ on object ⍺.
        rslt←⍺ ⎕WG ⎕IO⊃⍵    ⍝ get old value.
        rslt⊣⍺∘⎕WS do ⍵     ⍝ set new value and return old value.

    To  be  of  any  value, functions that don't return results must have useful
    side-effects.  In  other  words, they _do_ things, rather than indicate what
    the  value  of the function at a particular argument _is_. For more on this,

Technical notes:

This  coding  of [do] depends on the behaviour of primitive execute (⍎): Execute
of  a diamond-separated string is defined to return the evaluation of the right-
most segment as result.

    do←{                  ⍝ Apply no-result function.
        ⍎'⍺⍺ ⍵ ⋄ ⍵' ⋄ ⍺⍺

Notice that we must include an unquoted (and unvisited) ⍺⍺ within the braces, in
order  to  denote [do] as an operator and so consume the function operand to its

An  alternative  coding  might trap the expected value error. This has the minor
disadvantage that it disturbs any diagnostic message (⎕DM) and it appears to run
a little slower:

    do←{                  ⍝ Apply no-result function.
        6::⍵ ⋄ ⍺⍺ ⍵

Depressingly, [do] may be coded more simply and efficiently as a traditional op-

        ∇ RARG ← (FUNC DO) RARG
    [1]   FUNC RARG

but  perhaps  this  is to be expected as tradfns are generally better for "do-y"
programming,  while  D-functions excel at the "is-y" style (see the muse section


    'F1'∘⎕WS do 'Caption' 'Pugwash'

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