In the assert function {⍺←'assertion failure' ⋄ 0∊⍵:⍺ ⎕SIGNAL 8 ⋄ shy←0} from APLcart, what's the purpose of returning 0 as a shy result?

I think it might have to do with doing { ... ⋄ 'msg' assert x: ⍬ ⋄ ... } (as you can't have side-effect-only statements inside dfns)

So: if the assertion is true, return false (shyly), so that if the return value is used as a guard in a dfn, the guarded expression is skipped?

@rabbitgrowth Right, if used in a dfn, we need the 0-result so you can write assert x: and if used in a tradfn, we need it to be shy so assert x doesn't print anything.