Description
The second programming assigment in chapter 1 is
Write an ML function interp : stm→unit that “interprets” a program in this language. To write in a “functional” style – without [sml] assignment (:=) or arrays – maintain a list of (variable,integer) pairs, and produce new versions of this list at each AssignStm.
Appel suggests making two mutually recursive funcions:
val interpStm = fn : stm * table -> table
val interpExp = fn : exp * table -> int * table
These thread a table, a list of mappings from identifiers to values,
through the interpreter to keep track of the assignment statements:
type table = (id * int) list
Then you can write functions lookup and update to update the table
on assignments, and lookup the current value of an identifier.
val lookup = fn : table * id -> int
val update = fn : table * id * int -> table