to_s vs. to_str (and to_i/to_a/to_h vs. to_int/to_ary/to_hash) in Ruby

Note first that all of this applies to each pair of “short” (e.g. to_s/to_i/to_a/to_h) vs. “long” (e.g. to_str/to_int/to_ary/to_hash) coercion methods in Ruby (for their respective types) as they all have the same semantics.

They have different meanings. You should not implement to_str unless your object acts like a string, rather than just being representable by a string. The only core class that implements to_str is String itself.

From Programming Ruby (quoted from this blog post, which is worth reading all of):

[to_i and to_s] are not particularly strict: if an object has some kind of decent representation as a string, for example, it will probably have a to_s method… [to_int and to_str] are strict conversion functions: you implement them only if [your] object can naturally be used every place a string or an integer could be used.

Older Ruby documentation from the Pickaxe has this to say:

Unlike to_s, which is supported by almost all classes, to_str is normally implemented only by those classes that act like strings.

For example, in addition to Integer, both Float & Numeric implement to_int (to_i‘s equivalent of to_str) because both of them can readily substituted for an Integer (they are all actually numbers). Unless your class has a similarly tight relationship with String, you should not implement to_str.