Macro pest_consume::match_nodes

macro_rules! match_nodes {
    ($($x:tt)*) => { ... };
}

Pattern-match on Nodes with slice-like syntax and strong types..

See examples and crate-level documentation for usage.

Example usage:

let nodes: Nodes<_, _> = ...:
match_nodes!(nodes;
    [string(s), number(n)] => s.len() + n,
    [_, field(fs)..] => fs.filter(|f| f > 0).count(),
)

Syntax

The macro takes an expression followed by ;, followed by one or more branches separated by ,. Each branch has the form [$patterns] => $body. The body is an arbitrary expression. The patterns are a comma-seperated list of either $rule_name($binder) or just $binder, each optionally followed by .. to indicate a variable-length pattern.

How it works

match_nodes desugars rather straightforwardly into calls to the methods corresponding to the rules matched on. For example:

match_nodes!(input.into_children();
    [field(fields)..] => fields.count(),
    [string(s), number(n)] => s.len() + n,
)

desugars roughly into:

let nodes = { input.into_children() };
if ... { // check that all rules in `nodes` are the `field` rule
    let fields = nodes
        .map(|node| Self::field(node)) // Recursively parse children nodes
        ... // Propagate errors
    { fields.count() }
} else if ... { // check that the nodes has two elements, with rules `string` and `number`
    let s = Self::string(nodes.next().unwrap())?;
    let n = Self::number(nodes.next().unwrap())?;
    { s.len() + n }
} else {
    ... // error because we got unexpected rules
}

Matching raw nodes

Sometimes you may want to manipulate Nodes directly. For that, just omit a rule name when matching:

match_nodes!(input.into_children();
    [string(s), node] => { ... },
)

Here the first node will be parsed using the string rule, but the second node will be returned as-is. This also supports the .. syntax, which returns an iterator of Nodes.

This can come useful when pest_consume isn’t powerful enough for your use-case, for example if you want the ability to choose from multiple parsing functions for the same rule. This can usually be avoided by using some advanced features or tweaking the grammar, but if not you can always fall back to manipulating Nodes by hand.

Notes

The macro assumes that it is used within a consumer method, and uses Self::$method(...) to parse the input nodes. To use it outside a method, you can pass it the parser struct as follows (the angle brackets are mandatory):

match_nodes(<CSVParser>; nodes;
    ...
)

It also assumes it can return Err(...) in case of errors.