[lambda.git] / advanced_topics / calculator_improvements.mdwn

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We're going to make gradual improvements to the calculator we developed in [week7](/reader_monad_for_variable_binding).

##Original Calculator##

In a real programming application, one would usually start with a string that needs to be parsed and interpreted, such as:

        let x = 1 in let y = x + 2 in x * y

The parsing phase converts this to an "abstract syntax tree" (AST), which in this case might be:

        Let ('x', Constant 1,
                 Let ('y', Addition (Variable 'x', Constant 2),
                          Multiplication (Variable 'x', Variable 'y')))

Then the interpreter (or "evaluator") would convert that AST into an "expressed value": in this case, to the integer 3. We're not concerning ourselves with the parsing phase here, so we're just thinking about how to interpret expressions that are already in AST form.

The language we had in week 7 looked like this:

        type term =
          Constant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        ;;

and the evaluation function looked like this:

        let rec eval (t : term) (e: (char * int) list) = match t with
          Constant x -> x
        | Multiplication (t1, t2) -> (eval t1 e) * (eval t2 e)
        | Addition (t1, t2) -> (eval t1 e) + (eval t2 e)
        | Variable c ->
                (* lookup the value of c in the current environment
                   This will fail if c isn't assigned anything by e *)
                List.assoc c e
        | Let (c, t1, t2) ->
                (* evaluate t2 in a new environment where c has been associated
                   with the result of evaluating t1 in the current environment *)
                eval t2 ((c, eval t1 e) :: e)
        ;;


##Adding Booleans and Pairs##

Let's tweak this a bit.

First, let's abstract away from the assumption that our terms always evaluate to `int`s. Let's suppose they evaluate to a more general type, which might have an `int` payload, or might have, for example, a `bool` payload.

        type expressed_value = Int of int | Bool of bool;;

We'll add one boolean predicate, `Iszero`, and an `If...` construction.

Similarly, we might allow for some terms to express pairs of other terms:

        type expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value;;

We'd then want to add the ability to construct pairs, and extract their components.

We won't try here to catch any type errors, such as attempts to add a `bool` to an `int`, or attempts to check whether a `bool` iszero. Neither will we try here to monadize anything: these will be implementations of a calculator with all the plumbing exposed. What we will do is add more and more features to the calculator.

We'll switch over to using variable `g` for assignment functions, which is a convention many of you seem familiar with. As we mentioned a few times in [[week9]], for some purposes it's easier to implement environment or assignment functions as functions from `char`s to `int`s (or whatever variables are bound to), rather than as lists of pairs. However, we'll stick with this implementation for now. We will however abstract out the type that the variables are bound to. For now, we'll suppose that they're bound to the same types that terms can express.

        type bound_value = expressed_value;;
        type assignment = (char * bound_value) list;;

Here's where we should be now. We'll work with the language:

        type term =
          Intconstant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        | Iszero of term
        | If of (term * term * term)
        | Makepair of (term * term)
        | First of term
        ;;

Here is our evaluation function. We expand some of the clauses and rename a few variables for clarity. Our implementation should make it clear how to add additional constants or native predicates, such as a `Second` predicate for extracting the second element of a pair.

        let rec eval (t : term) (g : assignment) = match t with
          Intconstant x -> Int x
        | Multiplication (t1, t2) ->
                (* we don't handle cases where the subterms don't evaluate to Ints *)
                let Int i1 = eval t1 g
                in let Int i2 = eval t2 g
                (* Multiplication (t1, t2) should evaluate to an Int *)
                in Int (i1 * i2)
        | Addition (t1, t2) ->
                let Int i1 = eval t1 g
                in let Int i2 = eval t2 g
                in Int (i1 + i2)
        | Variable (var) ->
                (* we don't handle cases where g doesn't bind var to any value *)
                List.assoc var g
        | Let (var_to_bind, t2, t3) ->
                (* evaluate t3 under a new assignment where var_to_bind has been bound to
                   the result of evaluating t2 under the current assignment *)
                let value2 = eval t2 g
                in let g' = (var_to_bind, value2) :: g
                in eval t3 g'
        | Iszero (t1) ->
                (* we don't handle cases where t1 doesn't evaluate to an Int *)
                let Int i1 = eval t1 g
                (* Iszero t1 should evaluate to a Bool *)
                in Bool (i1 = 0)
        | If (t1, t2, t3) ->
                (* we don't handle cases where t1 doesn't evaluate to a boolean *)
                let Bool b1 = eval t1 g
                in if b1 then eval t2 g
                else eval t3 g
        | Makepair (t1, t2) ->
                let value1 = eval t1 g
                in let value2 = eval t2 g
                in Pair (value1, value2)
        | First (t1) ->
                (* we don't handle cases where t1 doesn't evaluate to a Pair *)
                let Pair (value1, value2) = eval t1 g
                in value1
        ;;

The complete code is available [here](/code/calculator/calc1.ml).

##Adding Function Values##

Now we want to add function values to our language, so that we can interpret (the abstract syntax trees of) expressions like this:

        let x = 1 in let f = lambda y -> y + x in apply f 2

What changes do we need to handle this?

We can begin with our language:

        type term =
          Intconstant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        | Iszero of term
        | If of (term * term * term)
        | Makepair of (term * term)
        | First of term
        | Lambda of (char * term)
        | Apply of (term * term)
        ;;

Next, we need to expand our stock of `expressed_value`s to include function values as well. How should we think of these? We've several times mentioned the issue of how to handle free variables in a function's body, like the `x` in `lambda y -> y + x`. We'll follow the usual functional programming standard for these (known as "lexical scoping"), which keeps track of what value `x` has in the function declaration's lexical environment. That shouldn't get shadowed by any different value `x` may have when the function value is later applied. So:

        let x = 1 in let f = lambda y -> y + x in let x = 2 in apply f 2

should evaluate to `3` not to `4`. To handle this, the function values we construct need to keep track of the present values of all free variables in the function's body. The combination of the function's body and the values of its free variables is called a "function closure." We'll implement these closures in a straightforward though inefficient way: we'll just stash away a copy of the assignment in effect when the function value is being constructed. Our function values also need to keep track of which of their variables are to be bound to the arguments they get applied to. All together, then, we need three pieces of information: which variables are to be bound to arguments, what the function's body is, and something that keeps track of the right values for the free variables in the function body. We'll pack this all together into an additional variant for our `expressed_value` type:

        type expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment;;

We'd like to define `bound_value`s and `assignment`s just as before:

        type bound_value = expressed_value;;
        type assignment = (char * bound_value) list;;

However, note that we have a recursive relation between these types: `expressed_value` is defined partly in terms of `assignment`, which is defined partly in terms of `bound_value`, which is equivalent to `expressed_value`. In OCaml one has to define such types using the following form:

        type bound_value = expressed_value
        and assignment = (char * bound_value) list
        and expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment;;

Now our evaluation function needs two further clauses to interpret the two new expression forms `Lambda (...)` and `Apply (...)`:

        let rec eval (t : term) (g : assignment) = match t with
        ...
        | Lambda (arg_var, t2) -> Closure (arg_var, t2, g)
        | Apply (t1, t2) ->
                (* we don't handle cases where t1 doesn't evaluate to a function value *)
                let Closure (arg_var, body, savedg) = eval t1 g
                in let value2 = eval t2 g
                (* evaluate body under savedg, except with arg_var bound to value2 *)
                in let savedg' = (arg_var, value2) :: savedg
                in eval body savedg'
        ;;

The complete code is available [here](/code/calculator/calc2.ml).

##Adding Recursive Functions##

There are different ways to include recursion in our calculator. First, let's imagine our language expanded like this:

        let x = 1 in letrec f = lambda y -> if iszero y then x else y * apply f (y - 1) in apply f 3

where the AST would be:

        Let ('x', Intconstant 1,
                Letrec ('f',
                        Lambda ('y',
                                If (Iszero (Variable 'y'),
                                        Variable 'x',
                                        Multiplication (Variable 'y',
                                                Apply (Variable 'f',
                                                        Addition (Variable 'y', Intconstant (-1)))))),
                        Apply (Variable 'f', Intconstant 3)))

Here is the expanded definition for our language type:

        type term =
          Intconstant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        | Iszero of term
        | If of (term * term * term)
        | Makepair of (term * term)
        | First of term
        | Lambda of (char * term)
        | Apply of (term * term)
        | Letrec of (char * term * term)
        ;;

Now consider what we'll need to do when evaluating a term like `Letrec ('f', Lambda (...), t2)`. The subterm `Lambda (...)` will evaluate to something of the form `Closure ('y', body, savedg)`, where `Variable 'f'` may occur free in `body`. What we'll want to do is to ensure that when `body` is applied, it's applied using not the assignment `savedg` but a modified assignment `savedg'` which binds `'f'` to this very function value. That is, we want to bind `'f'` not to:

        Closure ('y', body, savedg)

but instead to:

        let orig_closure = Closure ('y', body, savedg)
        in let savedg' = ('f', orig_closure) :: savedg
        in let new_closure = Closure ('y', body, savedg')
        in new_closure

Except, this isn't quite right. It's almost what we want, but not exactly. Can you see the flaw?

The flaw is this: inside `new_closure`, what is `'f'` bound to? It's bound by `savedg'` to `orig_closure`, which in turn leaves `'f'` free (or bound to whatever existing value it had according to `savedg`). This isn't what we want. It'll break if we need to make applications of `Variable 'f'` which recurse more than once.

What we really want is for `'f'` to be bound to `new_closure`, something like this:

        let rec new_closure = Closure ('y', body, ('f', new_closure) :: savedg)
        in new_closure

And as a matter of fact, OCaml *does* permit us to recursively define cyclical lists in this way. So a minimal change to our evaluation function would suffice:

        let rec eval (t : term) (g : assignment) = match t with
        ...
        | Letrec (var_to_bind, t2, t3) ->
                (* we don't handle cases where t2 doesn't evaluate to a function value *)
                let Closure (arg_var, body, savedg) = eval t2 g
        in let rec new_closure = Closure (arg_var, body, (var_to_bind, new_closure) :: savedg)
        in let g' = (var_to_bind, new_closure) :: g
                in eval t3 g'
        ;;
         
However, this is a somewhat exotic ability in a programming language, so it would be good to work out how to interpret `Letrec (...)` forms without relying on it.

If we implemented assignments as functions rather than as lists of pairs, the corresponding move would be less exotic. In that case, our `Let (...)` and `Letrec (...)` clauses would look something like this:


        | Let (var_to_bind, t2, t3) ->
                let value2 = eval t2 g
                in let g' = fun var -> if var = var_to_bind then value2 else g var
                in eval t3 g'
        ...
        | Letrec (var_to_bind, t2, t3) ->
                let Closure (arg_var, body, savedg) = eval t2 g
                in let rec savedg' = fun var -> if var = var_to_bind then Closure (arg_var, body, savedg') else savedg var
                in let g' = fun var -> if var = var_to_bind then Closure (arg_var, body, savedg') else g var
                in eval t3 g'
        ;;

and this is just a run-of-the-mill use of recursive functions. However, for this exercise we'll continue using lists of pairs, and work out how to interpret `Letrec (...)` forms using them.

The way we'll do this is that, when we bind a variable to a value, we'll keep track of whether the term was bound via `Let` or `Letrec`. We'll rely on that to interpret pairs of terms like these differently:

        Let ('f',
                Intconstant 1,
                Let ('f', Lambda ('y', Variable 'f')),
                ...)

        Let ('f',
                Intconstant 1,
                Letrec ('f', Lambda ('y', Variable 'f')),
                ...)

In the first case, an application of `Variable 'f'` to any argument should evaluate to `Int 1`; in the second case, it should evaluate to the same function closure that `Variable 'f'` evaluates to. We'll keep track of which way a variable was bound by expanding our `bound_value` type:

        type bound_value = Nonrecursive of expressed_value |
                Recursive_Closure of char * char * term * assignment
        and assignment = (char * bound_value) list
        and expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment;;


Since we're not permitting ourselves OCaml's ability to recursively define cyclical lists, we're not going to be able to update the saved assignment in a closure when that closure is recursively bound to a variable. Instead, we'll just make a note that variable `'f'` is supposed to be the recursively bound one---by binding it not to `Nonrecursive (Closure (arg_var, body, savedg))` but rather to `Recursive_Closure ('f', arg_var, body, savedg)`. We'll do the work to make the saved assignment recursive in the right way *later*, when we *evaluate* `Variable 'f'`. The result will look like this:

        let rec eval (t : term) (g : assignment) = match t with
        ...
        | Variable (var) -> (
                (* we don't handle cases where g doesn't bind var to any value *)
                match List.assoc var g with
          | Nonrecursive value -> value
          | Recursive_Closure (self_var, arg_var, body, savedg) as rec_closure ->
                          (* we update savedg to bind self_var to rec_closure here *)
              let savedg' = (self_var, rec_closure) :: savedg
              in Closure (arg_var, body, savedg')
        )
        | Let (var_to_bind, t2, t3) ->
                (* evaluate t3 under a new assignment where var_to_bind has been bound to
           the result of evaluating t2 under the current assignment *)
                let value2 = eval t2 g
                (* we have to wrap value2 in Nonrecursive *)
                in let g' = (var_to_bind, Nonrecursive value2) :: g
                in eval t3 g'
        ...
        | Lambda (arg_var, t2) -> Closure (arg_var, t2, g)
        | Apply (t1, t2) ->
                (* we don't handle cases where t1 doesn't evaluate to a function value *)
                let Closure (arg_var, body, savedg) = eval t1 g
                in let value2 = eval t2 g
                (* evaluate body under savedg, except with arg_var bound to Nonrecursive value2 *)
                in let savedg' = (arg_var, Nonrecursive value2) :: savedg
                in eval body savedg'
        | Letrec (var_to_bind, t2, t3) ->
                (* we don't handle cases where t2 doesn't evaluate to a function value *)
                let Closure (arg_var, body, savedg) = eval t2 g
        (* evaluate t3 under a new assignment where var_to_bind has been recursively bound to that function value *) 
                in let g' = (var_to_bind, Recursive_Closure (var_to_bind, arg_var, body, savedg)) :: g
                in eval t3 g'
        ;;

The complete code is available [here](/code/calculator/calc3.ml).

##Adding Mutable Cells##

Next, we'll add mutable cells (explicit-style mutation) to our calculator, as we did in [[week9]].

We'll add a few more syntactic forms to the language:

        type term =
          Intconstant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        | Iszero of term
        | If of (term * term * term)
        | Makepair of (term * term)
        | First of term
        | Lambda of (char * term)
        | Apply of (term * term)
        | Letrec of (char * term * term)
        | Newref of term
        | Deref of term
        | Setref of (term * term)
        ;;

And we now have to allow for `Mutcell`s as an additional kind of `expressed_value`. These are implemented as wrappers around an index into a `store`:

        type index = int;;

        type bound_value = Nonrecursive of expressed_value |
                Recursive_Closure of char * char * term * assignment
        and assignment = (char * bound_value) list
        and expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment | Mutcell of index;;

        type store = expressed_value list;;
 
Our evaluation function will now expect a `store` argument as well as an `assignment`, and will return an `expressed_value * store` pair:

        let rec eval (t : term) (g : assignment) (s : store) = match t with
          Intconstant x -> (Int x, s)
          ...
        | Variable (var) -> ((
                (* we don't handle cases where g doesn't bind var to any value *)
                match List.assoc var g with
          | Nonrecursive value -> value
          | Recursive_Closure (self_var, arg_var, body, savedg) as rec_closure ->
                          (* we update savedg to bind self_var to rec_closure here *)
              let savedg' = (self_var, rec_closure) :: savedg
              in Closure (arg_var, body, savedg')
        ), s)
          ...
        | Lambda (arg_var, t2) -> (Closure (arg_var, t2, g), s)
          ...

also, we'll need to be sure to thread the store argument through the evaluation of any subterms, as here:

        ...
        | Multiplication (t1, t2) ->
                (* we don't handle cases where the subterms don't evaluate to Ints *)
                let (Int i1, s') = eval t1 g s
                in let (Int i2, s'') = eval t2 g s'
                (* Multiplication (t1, t2) should evaluate to an Int *)
                in (Int (i1 * i2), s'')
        | Addition (t1, t2) ->
                let (Int i1, s') = eval t1 g s
                in let (Int i2, s'') = eval t2 g s'
                in (Int (i1 + i2), s'')
        ...
        | Let (var_to_bind, t2, t3) ->
                (* evaluate t3 under a new assignment where var_to_bind has been bound to
           the result of evaluating t2 under the current assignment *)
                let (value2, s') = eval t2 g s
                (* we have to wrap value2 in Nonrecursive *)
                in let g' = (var_to_bind, Nonrecursive value2) :: g
                in eval t3 g' s'
        | Iszero (t1) ->
                (* we don't handle cases where t1 doesn't evaluate to an Int *)
                let (Int i1, s') = eval t1 g s
                (* Iszero t1 should evaluate to a Bool *)
                in (Bool (i1 = 0), s')
        ...
        | Makepair (t1, t2) ->
                let (value1, s') = eval t1 g s
                in let (value2, s'') = eval t2 g s'
                in (Pair (value1, value2), s'')
        | First (t1) ->
                (* we don't handle cases where t1 doesn't evaluate to a Pair *)
                let (Pair (value1, value2), s') = eval t1 g s
                in (value1, s')
        ...
        | Apply (t1, t2) ->
                (* we don't handle cases where t1 doesn't evaluate to a function value *)
                let (Closure (arg_var, body, savedg), s') = eval t1 g s
                in let (value2, s'') = eval t2 g s'
                (* evaluate body under savedg, except with arg_var bound to Nonrecursive value2 *)
                in let savedg' = (arg_var, Nonrecursive value2) :: savedg
                in eval body savedg' s''
        | Letrec (var_to_bind, t2, t3) ->
                (* we don't handle cases where t2 doesn't evaluate to a function value *)
                let (Closure (arg_var, body, savedg), s') = eval t2 g s
        (* evaluate t3 under a new assignment where var_to_bind has been recursively bound to that function value *) 
                in let g' = (var_to_bind, Recursive_Closure (var_to_bind, arg_var, body, savedg)) :: g
                in eval t3 g' s'
        ...

The clause for `If (...)` is notable:

        ...
        | If (t1, t2, t3) ->
                (* we don't handle cases where t1 doesn't evaluate to a boolean *)
                let (Bool b1, s') = eval t1 g s
        (* note we thread s' through only one of the then/else clauses *)
                in if b1 then eval t2 g s'
                else eval t3 g s'
        ...

Now we need to formulate the clauses for evaluating the new forms `Newref (...)`, `Deref (...)`, and `Setref (...)`.

        ...
        | Newref (t1) ->
                let (value1, s') = eval t1 g s
                (* note that s' may be different from s, if t1 itself contained any mutation operations *)
                (* now we want to retrieve the next free index in s' *)
                in let new_index = List.length s'
                (* now we want to insert value1 there; the following is an easy but inefficient way to do it *)
                in let s'' = List.append s' [value1]
                (* now we return a pair of a wrapped new_index, and the new store *)
                in (Mutcell new_index, s'')
        | Deref (t1) ->
                (* we don't handle cases where t1 doesn't evaluate to a Mutcell *)
                let (Mutcell index1, s') = eval t1 g s
                (* note that s' may be different from s, if t1 itself contained any mutation operations *)
                in (List.nth s' index1, s')
        | Setref (t1, t2) ->
                (* we don't handle cases where t1 doesn't evaluate to a Mutcell *)
                let (Mutcell index1, s') = eval t1 g s
                (* note that s' may be different from s, if t1 itself contained any mutation operations *)
                in let (new_value, s'') = eval t2 g s'
                (* now we create a list which is just like s'' except it has new_value in index1 *)
                in let rec replace_nth lst m =
                        match lst with
                        | [] -> failwith "list too short"
                        | x::xs when m = 0 -> new_value :: xs
                        | x::xs -> x :: replace_nth xs (m - 1)
                in let s''' = replace_nth s'' index1
                (* we'll arbitrarily return Int 42 as the expressed_value of a Setref operation *)
                in (Int 42, s''')
        ;;

The complete code is available [here](/code/calculator/calc4.ml).

##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.

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.

To keep our codebase smaller, we'll implement mutable pairs instead of, not in addition to, the mutable cells from the previous section. Also, we'll leave out the immutable pairs we've been working with up to this point; in this implementation, all pairs will be mutable.

This implementation will largely parallel the previous one. Here are the differences. First, we remove the `Newref`, `Deref`, and `Setref` forms from the language. Our existing form `Makepair` will serve to create mutable pairs, and so will take over a role analogous to `Newref`. Our existing form `First` will take over a role analogous to `Deref`. We'll introduce one new form `Setfirst` that will take over a role analogous to `Setref`:

        type term =
          Intconstant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        | Iszero of term
        | If of (term * term * term)
        | Makepair of (term * term)
        | First of term
        | Lambda of (char * term)
        | Apply of (term * term)
        | Letrec of (char * term * term)
        | Setfirst of (term * term)
        ;;

Our `expressed_value` type changes in two ways: first, we eliminate the `Mutcell` variant added in the previous implementation. Instead, we now have our `Pair` variant wrap `index`es into the `store`:

        type index = int;;

        type bound_value = Nonrecursive of expressed_value |
                Recursive_Closure of char * char * term * assignment
        and assignment = (char * bound_value) list
        and expressed_value = Int of int | Bool of bool | Pair of index * index | Closure of char * term * assignment;;

        type store = expressed_value list;;

Finally, here are the changed or added clauses to the evaluation function:

        let rec eval (t : term) (g : assignment) (s : store) = match t with
        ...
        | Makepair (t1, t2) ->
                let (value1, s') = eval t1 g s
                in let (value2, s'') = eval t2 g s'
        (* now we want to retrieve the next free index in s'' *)
        in let new_index = List.length s''
        (* now we want to insert value1 and value2 there; the following is an easy but inefficient way to do it *)
        in let s''' = List.append s'' [value1; value2]
                in (Pair (new_index, new_index + 1), s''')
        | First (t1) ->
                (* we don't handle cases where t1 doesn't evaluate to a Pair *)
                let (Pair (index1, index2), s') = eval t1 g s
        (* note that s' may be different from s, if t1 itself contained any mutation operations *)
        in (List.nth s' index1, s')
        ...
    | Setfirst (t1, t2) ->
        (* we don't handle cases where t1 doesn't evaluate to a Pair *)
        let (Pair (index1, index2), s') = eval t1 g s
        (* note that s' may be different from s, if t1 itself contained any mutation operations *)
        in let (new_value, s'') = eval t2 g s'
        (* now we create a list which is just like s'' except it has new_value in index1 *)
        in let rec replace_nth lst m =
            match lst with
            | [] -> failwith "list too short"
            | x::xs when m = 0 -> new_value :: xs
            | x::xs -> x :: replace_nth xs (m - 1)
        in let s''' = replace_nth s'' index1
        in (Int 42, s''')
        ;;

Compare these to the clauses for `Newref`, `Deref`, and `Setref` in the previous implementation.

The complete code is available [here](/code/calculator/calc5.ml).

##Adding Implicit Mutation##

Next we implement implicit-style mutation, as we did in [[week9]]. Here we don't have any explicit reference cells or mutable pairs; we'll return pairs back to their original immutable form. Instead, all variables will have mutable bindings. New reference cells will be implicitly introduced by the `Let` form. They'll also be implicitly introduced by the `Apply` form---we didn't have function values on the table during the [[week9]] discussion, so this didn't come up then. The reason we introduce new reference cells when `Apply`ing a function value to arguments is that we don't want mutation of those arguments inside the body of the function to propagate out and affect the reference cell that may have supplied the argument. When we call functions in this implementation, we just want to supply them with *values*, not with the reference cells we may be drawing those values from. Below, after we discuss *aliases*, we'll consider another strategy, where function bodies are given the ability to mutate the reference cells implicitly associated with the arguments they're supplied.

Our language for the present implementation will be the language for the calculator with recursive functions, with one added syntactic form, `Change (...)`:

        type term =
          Intconstant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        | Iszero of term
        | If of (term * term * term)
        | Makepair of (term * term)
        | First of term
        | Lambda of (char * term)
        | Apply of (term * term)
        | Letrec of (char * term * term)
        | Change of (char * term * term)
        ;;

In the present implementation, we separate the roles of the `bound_value` and `expressed_value` types. As we discussed in [[week9]], our assignment will bind all variables to indexes in the store, and the latter will contain the `expressed_value`s that the variables evaluate to. A consequence of this is that our definitions of the `bound_value` and `expressed_value` types no longer need to be mutually recursive:
         
        type index = int;;
         
        type bound_value = index;;
        type assignment = (char * bound_value) list;;
        type expressed_value = Int of int | Bool of bool | Pair of expressed_value * expressed_value | Closure of char * term * assignment;;
         
        type store = expressed_value list;;

Our evaluation function still interacts with a `store` argument in much the same way it did with explicit-style mutation. The clause for `Variable ...` works differently, because all `expressed_value`s now need to be retrieved from the `store`:
         
        let rec eval (t : term) (g : assignment) (s : store) = match t with
        ...
        | Variable (var) ->
                (* we don't handle cases where g doesn't bind var to any value *)
                let index = List.assoc var g
                (* get value stored at location index in s *)
                in let value = List.nth s index
                in (value, s)
        ...

So this clause takes over the roles that were separately played by `Variable` and `Deref` in the calculator with mutable cells. The role played by `Newref` is absorbed into `Let`, `Letrec`, and `Apply`:

        ...
        | Let (var_to_bind, t2, t3) ->
                let (value2, s') = eval t2 g s
                (* note that s' may be different from s, if t2 itself contained any mutation operations *)
                (* get next free index in s' *)
                in let new_index = List.length s'
                (* now we want to insert value2 there; the following is an easy but inefficient way to do it *)
                in let s'' = List.append s' [value2]
                (* bind var_to_bind to location new_index in the store *)
                in let g' = ((var_to_bind, new_index) :: g)
                in eval t3 g' s''
        ...
        | Apply (t1, t2) ->
                (* we don't handle cases where t1 doesn't evaluate to a function value *)
                let (Closure (arg_var, body, savedg), s') = eval t1 g s
                in let (value2, s'') = eval t2 g s'
                (* evaluate body under savedg, except with arg_var bound to a new location containing value2 *)
                in let new_index = List.length s''
                in let s''' = List.append s'' [value2]
                in let savedg' = (arg_var, new_index) :: savedg
                in eval body savedg' s'''
        ...

`Letrec` requires some reworking from what we had before. Earlier, we resorted to a `Recursive_Closure` variant on `bound_value`s because it gave us a non-exotic way to update the `savedg` component of a `Closure` to refer to a `new_closure` that contained that very updated `savedg`. Now that we we've got a mutation-supporting infrastructure in place, we can do this directly, without needing the unwieldy `Recursive_Closure` wrapper:

        ...
        | Letrec (var_to_bind, t2, t3) ->
                (* we don't handle cases where t2 doesn't evaluate to a function value *)
                let (Closure (arg_var, body, savedg), s') = eval t2 g s
                in let new_index = List.length s'
                in let savedg' = (var_to_bind, new_index) :: savedg
                in let new_closure = Closure (arg_var, body, savedg')
                in let s'' = List.append s' [new_closure]
                in let g' = (var_to_bind, new_index) :: g
                in eval t3 g' s''
        ...

Finally, here is the clause for `Change (...)`, which takes over the role earlier played by `Setref`: 

        ...
        | Change (var, t2, t3) ->
                (* we don't handle cases where g doesn't bind var to any value *)
                let index = List.assoc var g
                in let (value2, s') = eval t2 g s
                (* note that s' may be different from s, if t2 itself contained any mutation operations *)
                (* now we create a list which is just like s' except it has value2 at index *)
                in let rec replace_nth lst m =
                        match lst with
                        | [] -> failwith "list too short"
                        | x::xs when m = 0 -> value2 :: xs
                        | x::xs -> x :: replace_nth xs (m - 1)
                in let s'' = replace_nth s' index
                (* evaluate t3 using original assignment function and new store *)
                in eval t3 g s''
        ;;

Note that because the `savedg` component of a `Closure` keeps track of which `index`es in the store free variables were bound to, the values at those `index`es can later be changed, and later applications of the `Closure` will use the changed values.

The complete code is available [here](/code/calculator/calc6.ml).

##Adding Aliasing and Passing by Reference##

        type term =
          Intconstant of int
        | Multiplication of (term * term)
        | Addition of (term * term)
        | Variable of char
        | Let of (char * term * term)
        | Iszero of term
        | If of (term * term * term)
        | Makepair of (term * term)
        | First of term
        | Lambda of (char * term)
        | Apply of (term * term)
        | Letrec of (char * term * term)
        | Change of (char * term * term)
        | Alias of (char * char * term)
        | Applyalias of (term * char)
        ;;
 
        let rec eval (t : term) (g : assignment) (s : store) = match t with
        ...
        | Alias (var_to_bind, orig_var, t3) ->
                (* we don't handle cases where g doesn't bind orig_var to any value *)
                let index = List.assoc orig_var g
                (* bind var_to_bind to the same index in the store *)
                in let g' = ((var_to_bind, index) :: g)
                in eval t3 g' s
        | Applyalias (t1, var) ->
                (* we don't handle cases where t1 doesn't evaluate to a function value *)
                let (Closure (arg_var, body, savedg), s') = eval t1 g s
                (* we don't handle cases where g doesn't bind var to any value *)
                in let index = List.assoc var g
                (* evaluate body under savedg, except with arg_var bound to existing index *)
                in let savedg' = (arg_var, index) :: savedg
                in eval body savedg' s'
        ;;

The complete code is available [here](/code/calculator/calc7.ml).