I use the approach you mention, restoring stream position, in my ParrotTalk code, both in Pharo and in Java. I am not peeking more than one character but I had to implement peek in Java and your code example is how I did this code. - HH On Thu, Nov 2, 2017 at 04:59, Prof. Andrew P. Black <[black@cs.pdx.edu]("mailto:black@cs.pdx.edu")> wrote:
I sometimes find, when working with a stream, that I need to "peek ahead" a few items, without disturbing the state of the stream. The message stream peek gives me the next item, but doesnât generalize to multiple items, So I end up writing code like this:
findSomething | savedPosition result | savedPosition := stream position. stream position: start. "code involving various manipulations of stream, including stream next" result := stream position + 1. stream position: savedPosition. ^ result
This code involves at least two temps, is not very easy to read, and risks not resetting the stream properly if I do a return.
It seems to me that a better solution would be what Kent Beck calls an "execute around" method:
findSomething ^ stream unchangedDuring: [ :s | "code involving various manipulations of stream, including stream next"stream position + 1 ]
Iâm not sure that I like the name #unchangedDuring:; perhaps you can think of a better one?
The implementation of this would be
PositionableStream >> #unchangedDuring: aBlock "execute aBlock with self as argument. Ensure that when this method returns, I still have my current state."
| savedPosition |savedPosition := self position. ^ [ aBlock value: self ] ensure: [ self position: savedPosition ]
or perhaps
PositionableStream >> #unchangedDuring: aBlock "execute aBlock with self as argument. Ensure that when this method returns, I still have my current state." ^ [ aBlock value: self copy ]
which might be better in the case that the stream is a file stream and aBlock causes the buffer to be refreshed from the disk.
- Do other Smalltalks have a method like this? - Does it already exist in Pharo, somewhere that Iâve failed to look? - Would it be a worthwhile addition to Pharo? - What would be a good name?