Ruby: Inconsistencies

This is a growing list:

• String#concat
• Should be String#concat!
• I guess it's meant to resemble #insert, #delete, #fill, #replace, #clear, etc., but I wish those had "!" too, like #compact!, #reject!, #slice!, etc. There should be 2 versions of all these, but because there is no "!", the non-mutating versions can never be.
• Hash#update is a synonym for Hash#merge!
• I have to remember which one has the "!"
• Hash#invert does *not* mutate. How do I remember all this?
• Even worse, Set#merge (no "!") mutates, unlike Hash#merge.
• Set#add? and Set#delete?
• These mutate.
• s.chomp!.upcase
• Can fail, since String#chomp! can return nil
• Array#fetch(i) (and Hash#fetch(k)) can raise IndexError
• Should be Array#fetch!(i)
• Block and multi-arg version could drop the "!"
• String#each does not exist in Ruby1.9, while #each_char does not exist in 1.8
• We cannot write forward-compatible code!
• Soln: s.split("")
• '0x10'.hex and '0x10'.oct are same, but
• '010'.hex and '010'.oct are different
• Given: a = [1,2,3]
• a[2] == 3
• a[3] == nil
• a[4] == nil
• a[2,1] == [3] (and a[2,2] == [3] as well)
• but a[3,1] == []
• while a[4,1] == nil
• For Arrays
• a[x,y] = [nil] substitutes the slice
• a[x,y] = z     substitutes [z], but ...
• a[x,y] = nil   deletes the slice! (fixed in 1.9)
• inspect/to_s
• There used to be a clear distinction, like Python's __repr__/__str__, but 1.9 often (though not in all cases) erases that useful distinction.

• Array   Float   Integer   String 

• Those are Kernel functions, not constants.
• ...

Also see Ruby Best Practices, a great book.