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0160-if-let.md

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Summary

Introduce a new if let PAT = EXPR { BODY } construct. This allows for refutable pattern matching without the syntactic and semantic overhead of a full match, and without the corresponding extra rightward drift. Informally this is known as an "if-let statement".

Motivation

Many times in the past, people have proposed various mechanisms for doing a refutable let-binding. None of them went anywhere, largely because the syntax wasn't great, or because the suggestion introduced runtime failure if the pattern match failed.

This proposal ties the refutable pattern match to the pre-existing conditional construct (i.e. if statement), which provides a clear and intuitive explanation for why refutable patterns are allowed here (as opposed to a let statement which disallows them) and how to behave if the pattern doesn't match.

The motivation for having any construct at all for this is to simplify the cases that today call for a match statement with a single non-trivial case. This is predominately used for unwrapping Option<T> values, but can be used elsewhere.

The idiomatic solution today for testing and unwrapping an Option<T> looks like

match optVal {
    Some(x) => {
        doSomethingWith(x);
    }
    None => {}
}

This is unnecessarily verbose, with the None => {} (or _ => {}) case being required, and introduces unnecessary rightward drift (this introduces two levels of indentation where a normal conditional would introduce one).

The alternative approach looks like this:

if optVal.is_some() {
    let x = optVal.unwrap();
    doSomethingWith(x);
}

This is generally considered to be a less idiomatic solution than the match. It has the benefit of fixing rightward drift, but it ends up testing the value twice (which should be optimized away, but semantically speaking still happens), with the second test being a method that potentially introduces failure. From context, the failure won't happen, but it still imposes a semantic burden on the reader. Finally, it requires having a pre-existing let-binding for the optional value; if the value is a temporary, then a new let-binding in the parent scope is required in order to be able to test and unwrap in two separate expressions.

The if let construct solves all of these problems, and looks like this:

if let Some(x) = optVal {
    doSomethingWith(x);
}

Detailed design

The if let construct is based on the precedent set by Swift, which introduced its own if let statement. In Swift, if let var = expr { ... } is directly tied to the notion of optional values, and unwraps the optional value that expr evaluates to. In this proposal, the equivalent is if let Some(var) = expr { ... }.

Given the following rough grammar for an if condition:

if-expr     = 'if' if-cond block else-clause?
if-cond     = expression
else-clause = 'else' block | 'else' if-expr

The grammar is modified to add the following productions:

if-cond = 'let' pattern '=' expression

The expression is restricted to disallow a trailing braced block (e.g. for struct literals) the same way the expression in the normal if statement is, to avoid ambiguity with the then-block.

Contrary to a let statement, the pattern in the if let expression allows refutable patterns. The compiler should emit a warning for an if let expression with an irrefutable pattern, with the suggestion that this should be turned into a regular let statement.

Like the for loop before it, this construct can be transformed in a syntax-lowering pass into the equivalent match statement. The expression is given to match and the pattern becomes a match arm. If there is an else block, that becomes the body of the _ => {} arm, otherwise _ => {} is provided.

Optionally, one or more else if (not else if let) blocks can be placed in the same match using pattern guards on _. This could be done to simplify the code when pretty-printing the expansion result. Otherwise, this is an unnecessary transformation.

Due to some uncertainty regarding potentially-surprising fallout of AST rewrites, and some worries about exhaustiveness-checking (e.g. a tautological if let would be an error, which may be unexpected), this is put behind a feature gate named if_let.

Examples

Source:

if let Some(x) = foo() {
    doSomethingWith(x)
}

Result:

match foo() {
    Some(x) => {
        doSomethingWith(x)
    }
    _ => {}
}

Source:

if let Some(x) = foo() {
    doSomethingWith(x)
} else {
    defaultBehavior()
}

Result:

match foo() {
    Some(x) => {
        doSomethingWith(x)
    }
    _ => {
        defaultBehavior()
    }
}

Source:

if cond() {
    doSomething()
} else if let Some(x) = foo() {
    doSomethingWith(x)
} else {
    defaultBehavior()
}

Result:

if cond() {
    doSomething()
} else {
    match foo() {
        Some(x) => {
            doSomethingWith(x)
        }
        _ => {
            defaultBehavior()
        }
    }
}

With the optional addition specified above:

if let Some(x) = foo() {
    doSomethingWith(x)
} else if cond() {
    doSomething()
} else if other_cond() {
    doSomethingElse()
}

Result:

match foo() {
    Some(x) => {
        doSomethingWith(x)
    }
    _ if cond() => {
        doSomething()
    }
    _ if other_cond() => {
        doSomethingElse()
    }
    _ => {}
}

Drawbacks

It's one more addition to the grammar.

Alternatives

This could plausibly be done with a macro, but the invoking syntax would be pretty terrible and would largely negate the whole point of having this sugar.

Alternatively, this could not be done at all. We've been getting alone just fine without it so far, but at the cost of making Option just a bit more annoying to work with.

Unresolved questions

It's been suggested that alternates or pattern guards should be allowed. I think if you need those you could just go ahead and use a match, and that if let could be extended to support those in the future if a compelling use-case is found.

I don't know how many match statements in our current code base could be replaced with this syntax. Probably quite a few, but it would be informative to have real data on this.