Wuffs functions have one of three effects: pure, impure and coroutine. Pure means that the function has no side effects - it does not change any observable state. For example, the body of a pure function (with a this receiver) cannot assign to a this.foo field. The other two categories may have side effects, with coroutines also being able to suspend and resume.

Impure functions are marked with a ! at their definition and at call sites. Coroutines are similarly marked, with a ?. Pure functions have no mark.

For those used to C/C++ syntax, in Wuffs, the unary not operator is spelled not instead of !, and Wuffs has no ternary operator like C/C++'s ?:.

Sub-expressions in Wuffs must be pure. Only the outermost function call can have a ! or ? mark. You can't write:

foo!(bar(), baz!(), qux)

Instead, you have to explicitly split it.

b = baz!()
foo!(bar(), b, qux)

For similar reasons, x += 1 is a statement in Wuffs, not an expression. This avoids the ambiguous order of execution in C/C++'s x = x++, which is actually undefined behavior.