A data structure to memoize the result of the execution of a sequence of commands.

As long as the sequence of commands is identical to the one memoized, we can avoid executing them. As soon as there is one command that differs from the memoized sequence, then we should discard all remaining memoized results.

Usage

Let's store the result of some commands (we alternate between memo and memo' to avoid recursive definitions)

>>> import Prelude hiding (lookup)
>>> memo' = storeResult "x=1" "" empty :: CmdMemoizer String String
>>> memo = storeResult "y=2" "" memo'
>>> memo' = storeResult "x+y" "3" memo

Suppose there are no more commands in the sequence. Now we want to execute that sequence of commands again but some commands may have changed in between. We use memoized commands as long as all commands are equal to the previous ones:

>>> memo = restart memo'
>>> lookup "x=1" memo
Just ""

Since the command was memoized, we avoid executing is again. Now suppose the command "y=2" was replaced by "y=3"

>>> memo' = nextCmd memo
>>> lookup "y=3" memo'
Nothing

Since the command was not memoized, we have to execute it:

>>> memo = storeResult "y=3" "" memo'

Now none of the subsequent commands will use the memoized version:

>>> memo' = nextCmd memo
>>> lookup "x+y" memo'
Nothing