You can perform just writes inside the block. If you want atomicity with reads in the middle, you need to use WATCH/MULTI/EXEC that uses an optimistic locking algorithm.

so for this you would really just want to do: APPEND foo d

or for more complex situations you could do use WATCH instead of MULTI so that you can get the intermediate results. so for your example:

WATCH foo GET foo ->value //calculate new_value MULTI SET foo new_value EXEC

now if at any other operation (not part of this connection) modifies foo then the transaction will be aborted.

GET foo -> value //application logic to update value GETSET foo new_value ->prev

then prev will contain the value of foo at the time it was set, so you can then use application logic to roll back or redo the transaction