{ } — Dfn / Dop{…} defines a function with arguments ⍺ and ⍵. Names follow lexical scope.
{⍵×2}3 ⍝ 6
2{⍺+⍵}3 ⍝ 5
The first result-producing non-assignment expression returns. Guards choose a result; a final assignment returns silently.
{⍵<0:-⍵ ⋄ ⍵}¯3 ⍝ 3
{x←⍵+1}3 ⍝ 4
Using ⍶ defines a monadic operator; ⍹ makes it dyadic.
twice←{⍶ ⍶ ⍵} ⋄ (-twice)3 ⍝ 3
See defaults, recursion and error guards.