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.