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crystal-lang

What is Crystal's Char::Reader defined as a Struct and not a Class?

Char::Reader is defined in the standard library as a Struct. What was the reasoning behind choosing a Struct instead of a Class?

Solution

  • I think that was choosen for performance reasons. According to the docs:

    A struct is mostly used for performance reasons to avoid lots of small memory allocations when passing small copies might be more efficient.

    So how do you choose between a struct and a class? The rule of thumb is that if no instance variable is ever reassigned, i.e. your type is immutable, you could use a struct, otherwise use a class.

    Char::Reader is mutable (because has instance variables reassigned), but it seems to be safe enough to do what the compiler internals do even if reader is a struct:

    reader = Char::Reader.new(pattern)
while reader.has_next?
  char = reader.current_char
  reader = check_char reader, char
  reader.next_char
end