Here are some things about Io that surprised me as I came to learn it. As background, I have experience in JavaScript, Lua, and Ruby.

t1 := Date now
t2 := Date now
# oops; t1 just got mutated
# must use Date clone now

• Bad docs for Number asString

• Number asString, first param oddly named

• Where's sprintf? Any convenient way of formatting numbers as strings?

• Why is Protos Addons not the same as Protos protos last? (discussed on mailing list)

• The 'magic' updateSlot that operates on self and the 'magic' forward that forwards messages to self conflicts mentally with no 'magic' setSlot. It's hard to know when you must use self in a method, and when it's OK not to. (Especially with do(...) not requiring it.)

thisLocalContext slotNames contains( "forward" ) == false
thisLocalContext protos isEmpty == true
# ... so how does it forward messages to self?

Steve Dekorte says that there's an optimization that hard-looks in Locals instead of via protos.

• Frustrating amount of no-documentation methods. Not just no examples, but not even a summary description.

• The word 'context' is confusing. Why not just 'object'?

• There is Sequence endsWithSeq, where is startsWithSeq?

• In Ruby, Array#empty? tests if it's empty. A few times I've done myList empty and been weirded out when it never returns false.

• foo or false results in true, not foo. Makes it harder to make guard statements than Ruby/JS/Lua.

• foo := Object clone; foo true := foo; foo true #=> true Why isn't this 'foo'?

• "foo\nbar" escape #=> Exception: 'escape' cannot be called on an immutable Sequence Why would I expect escape to mutate the sequence instead of returning a copy? Why shouldn't I expect slice() to mutate the sequence? How do I know which Sequence methods will throw an exception on an Immutable Sequence?

• There's no way to write an operator with higher precendence than message passing, e.g. foo ! bar println

• += doesn't work on Mutable sequences; gives unhelpful exception