Default Left Argument

The special  syntax: ⍺←expr is used to give a default value to the left argument if a dfn is called monadically. For example:

      root←{      ⍝ ⍺th root
          ⍺←2     ⍝ default to sqrt
          ⍵*÷⍺
      }

The expression to the right of  ⍺← is evaluated only if its dfn is called with no left argument.

Note that the assignment ⍺←⊢ allows an ambivalent function to call an ambivalent sub-function. For example in:

      foo←{
         ⍺←⊢
         ⍺ goo ⍵
      }

If foo is given a left argument, this is passed to goo. Otherwise, ⍺ is assigned ⊢ and the last line is ⊢ goo ⍵, which is a monadic call on goo followed by the ⊢ (Right) of the result of goo, which is the same value.