ECMAScript proposal: do expressions
Status
This proposal is in stage 1 of the TC39 process.
Motivation
- expression-oriented programming one of the great advances of FP
- expressions plug together like legos, making more malleable programming experience in-the-small
Examples
Write in an expression-oriented style, scoping variables as locally as possible:
let x = do {
let tmp = f();
tmp * tmp + 1
};
Use conditional statements as expressions, instead of awkward nested ternaries:
let x = do {
if (foo()) { f() }
else if (bar()) { g() }
else { h() }
};
Especially nice for templating languages like JSX:
return (
<nav>
<Home />
{
do {
if (loggedIn) {
<LogoutButton />
} else {
<LoginButton />
}
}
}
</nav>
)
Tennant's Correspondence Principle
- key refactoring principles:
do { <expr>; }equivalent to<expr>(do { <stmt> };)equivalent to{ <stmt> }
- this semantic transparency is demonstrated by the semantics:
- Return the result of evaluating Body.
Further considerations
How to avoid either parsing conflict in statement context with do-while, or dangling-else type of ambiguity:
do do f(); while (x);
I have several alternatives I intend to explore here.