Assigning to an expression with dfns

[OSRK]

      v←0 0 
      myfunc←⊃
      (2⊃v)←3
      (2 myfunc v)←2
SYNTAX ERROR
      (2 myfunc v)←2
       ∧
      myfunc2←{⍺⊃⍵}
      (2 myfunc2 v)←2
SYNTAX ERROR
      (2 myfunc2 v)←2
       ∧

Why?

How to create a kind of custom sub-select function that could be used on the left side of assignment?

[JohnS|Dyalog]

The selective assignment construct allows only a subset of primitive functions inside its parentheses. These primitive functions are being applied to an internal structure representing the indices of the subject array, which is meaningless to more general functions, such as the dfn {⍺⊃⍵} in your myfunc2 example.

PS: for selective specification of array items, try the primitive @ operator.

      3@2 ⊢0 0
0 3

John