Conditional parsing

During parsing we often want to decide at runtime what to parse next, e.g., certain fields might only be set if a previous field has a certain value, or the type for the next field might be known dynamically from a previous field.

We can specify that a field should only be parsed if a condition is met.

type Integer = unit {
    width: uint8 &requires=($$ != 0 && $$ < 5);
    u8 : uint8  if (self.width == 1);
    u16: uint16 if (self.width == 2);
    u32: uint32 if (self.width == 3);
    u64: uint64 if (self.width == 4);
};

Alternatively we can express this with a unit switch statement.

type Integer = unit {
    width: uint8 &requires=($$ != 0 && $$ < 5);
    switch (self.width) {
        1 -> u8: uint8;
        2 -> u16: uint16;
        3 -> u32: uint32;
        4 -> u64: uint64;
    };
};

In both cases the unit will include all fields, both set and unset. Once can query whether a field has been set with ?., e.g.,

on Integer::%done {
    if (self?.u8) { print "u8 was extracted"; }
}

Often parsing requires examining input and dynamically choosing a matching parser from the input. Spicy models this with lookahead parsing which is explained in a separate section.