[[!toc]]
-In class on November 22, we just reviewed mutation and aliasing, especially in relation to the Groenendijk, Stockhof and Veltman paper. Below, we bring in more material. This takes the form of making gradual improvements to the calculator we developed in [week7](/reader_monad_for_variable_binding). Part of what we do below is a review of the mutation techniques developed in [[Week9]]; but we also do other things we haven't discussed in class, like defining new complex functions in our calculator.
+In class on November 22, we just reviewed mutation and aliasing, especially in relation to the Groenendijk, Stokhof and Veltman paper. Below, we bring in more material. This takes the form of making gradual improvements to the calculator we developed in [week7](/reader_monad_for_variable_binding). Part of what we do below is a review of the mutation techniques developed in [[Week9]]; but we also do other things we haven't discussed in class, like defining new complex functions in our calculator.
##Original Calculator##
##Adding Mutable Pairs##
-Suppose we wanted to work with pairs where we could mutate either component of the pair. Well, we've already given ourselves pairs, and mutable cells, so we could just work here with pairs of mutable cells. But it might sometimes be more wieldy to work with a structure that fused these two structures together, to give us a mutable pair. With the mutable pair, we wouldn't ask for the first element, and then apply `Deref` to it to get the value it then temporarily contains. Instead, asking for the first element would *constitute* asking for the value the mutable pair then temporarily contains in its first position.
+Suppose we wanted to work with pairs where we could mutate either component of the pair. (If you've got a copy of *The Seasoned Schemer*, which we recommended for the seminar, see the discussion of mutable lists at pp. 143-153.)
+
+Well, we've already given ourselves pairs, and mutable cells, so we could just work here with pairs of mutable cells. But it might sometimes be more wieldy to work with a structure that fused these two structures together, to give us a mutable pair. With the mutable pair, we wouldn't ask for the first element, and then apply `Deref` to it to get the value it then temporarily contains. Instead, asking for the first element would *constitute* asking for the value the mutable pair then temporarily contains in its first position.
This means a mutable pair is an interesting hybrid between explicit-style and implicit-style mutation. Looked at one way, it's just a generalization of an explicit mutable cell: it's just that where the mutable cells we implemented before were boxes with only one position, now we have boxes with two positions. Looked at another way, though, mutable pairs are similar to implicit-style mutation: for we don't have separate ways of referring to the first position of the mutable pair, and its dereferenced value. Peeking at the first position *just will be* peeking at its current dereferenced value.