Gotchas

? and _ — context-sensitive symbols

? has two meanings, _ has two meanings:

Context	Syntax	Meaning
Type annotation	x:_	Any/unknown type (wildcard)
Match expression	?x{...}	Match on value
Prefix ternary	?=x 0 10 20	Conditional expression
Match wildcard	_:"default"	Catch-all arm

_ means “don’t care” in both type annotations and match patterns. ? is always a conditional/match operator.

ilo

f x:_>t;?x{n v:"number";t v:"text";_:"other"}

Here _ after : is the any type, ? starts a match, and _: in the match is the wildcard arm.