Hi Eric,
On Thu, 6 Aug 2020, Eric Gade wrote:
Hello,
What is the expected behavior for a stream that is at position 1 when you send #peekBack? Intuitively I'd expect to get the first element of the underlying collection as a response. But instead I am getting nil. Here is an example that is failing:
elem := #(1 2 3 4). stream := ReadStream on: elem. first := stream next. first = 1. back := stream peekBack. back = first.
The last message is currently responding false. In Pharo this returns true, so there are differences.
The currently implementation of PositionableStream >> #peekBack is:
peekBack "Return the element at the previous position, without changing position. Use indirect messages in case self is a StandardFileStream." | element | self position = 0 ifTrue: [self errorCantGoBack]. self position = 1 ifTrue: [self position: 0. ^ nil]. self skip: -2. element := self next. self skip: 1. ^ element
I can see the nil being returned there explicitly, so that's "where it's happening." Should this be the case though?
The previous, probably original implmenetation of #peekBack used to send #oldBack to the stream. IIRC #oldBack was based on the idea that the position of a stream is an index of a sequence where #next means +1 to that index while #back means -1 to that index. Following that logic, you have to #skip: -2 and send #next to get the element -1 to position.
#oldBack has been removed but the behavior of #peekBack is presumably the same as it was before. Some ancient but now external code may rely on #peekBack but it's not very likely such code would work in the current Trunk. #peekBack has no real users in the Trunk only a test remembers what it used to do. So, I think it's a good time to change its behavior to be based on #back.
Levente
Thanks
-- Eric