Type inference

OK, now that we understand the calculator1 example, let's look at some of the shorthands that LALRPOP offers to make it more concise. This code is found in the calculator2 demo.

To start, let's look at the definition of Term we saw before:

pub Term: i32 = {
    <n:Num> => n,
    "(" <t:Term> ")" => t,
};

The action code here is somewhat interesting. In both cases, it's not doing any new work, it's just selecting a value that was produced by another nonterminal. This turns out to be pretty common. So common, in fact, that LALRPOP offers some shorthand notation for it. Here is the definition of Term from the calculator2 demo:

pub Term = { Num, "(" <Term> ")" };

Here, we have no action code at all. If there is no action code, LALRPOP synthesizes action code which just takes the value of the things being matched. In the case of the first alternative, Num, there is only one thing being matched, so that means that Term will produce the same value as the Num we parsed, whatever that was.

In the case of the second alternative, "(" <Term> ")", there are three things being matched. Here we use the angle brackets to select which item(s) we want to take the value of --- we selected only one, so the result is that we take the value of the Term we parsed. If we selected more than one, the result would be a tuple of all the selected items. If we did not select any (i.e., "(" Term ")"), the result would be a tuple of all the items, and hence the result would be of type (&'input str, i32, &'input str).

Speaking of types, you may have noticed that Term has no type annotation. Since we didn't write out own action code, we can omit the type annotation and let LALRPOP infer it for us. In this case, LALRPOP can see that Term must have the same type as Num, and hence that the type must be i32.

OK, let's look at the definition of Num we saw before from calculator1:

Num: i32 = <s:r"[0-9]+"> => i32::from_str(s).unwrap();

This definition too can be made somewhat shorter. In calculator2, you will find:

Num: i32 = r"[0-9]+" => i32::from_str(<>).unwrap();

Here, instead of giving the regular expression a name s, we modified the action code to use the funky expression <>. This is a shorthand that says "synthesize names for the matched values and insert a comma-separated list here". In this case, there is only one matched value, r"[0-9]+", and it produces a &'input str, so LALRPOP will insert a synthetic variable for that value. Note that we still have custom action code, so we still need a type annotation.

To control what values are selected when you use the <> expression in your action code, you can use angle brackets as we saw before. Here are some examples of alternatives and how they are expanded to give you the idea:

Alternative Equivalent to
A => bar(<>) <a:A> => bar(a)
A B => bar(<>) <a:A> <b:B> => bar(a, b)
A B => (<>) <a:A> <b:B> => (a, b)
<A> B => bar(<>) <a:A> B => bar(a)
<p:A> B => bar(<>) <p:A> B => bar(p)
<A> <B> => bar(<>) <a:A> <b:B> => bar(a, b)
<p:A> <q:B> => bar(<>) <p:A> <q:B> => bar(p, q)
<p:A> B => Foo {<>} <p:A> B => Foo {p:p}
<p:A> <q:B> => Foo {<>} <p:A> <q:B> => Foo {p:p, q:q}

The <> expressions also works with struct constructors (like Foo {...} in examples above). This works out well if the names of your parsed values match the names of your struct fields.