# `⊘` — Valences

Keys: `Alt-o /`

`f⊘g` calls `f` when it has one argument, and `g` when it has two.
Either side can be a dfn with any number of statements.

``` bpl
f←{⍵×2}⊘{⍺+⍵}
f 5                ⍝ 10
3 f 5              ⍝ 8
```

A dfn can also serve both cases, because an absent `⍺` drops out of the
expression that uses it: `{⍺-⍵}3` is `¯3`. See
[dfns](../dfns.ipynb#arguments).

An array operand acts as a constant function. `⍺ 0⊘1 ⍵` uses that to
test for a left argument. It gives `1` in a call with two arguments and
`0` in a call with one.

``` bpl
{⍺ 0⊘1 ⍵}5         ⍝ 0
3{⍺ 0⊘1 ⍵}5        ⍝ 1
```

APL difference: Dyalog has no Valences operator. BQN writes it as `⊘`
too.