(set-box! ycell 3)
(+ x (unbox ycell)))
-When dealing with explicit-style mutation, there's a difference between the types and values of `ycell` and `!ycell` (or `(unbox ycell)`). The former has the type `int ref`: the variable `ycell` is assigned a reference cell that contains an `int`. The latter has the type `int`, and has whatever value is now stored in the relevant reference cell. In an implicit-style framework though, we only have the resources to refer to the contents of the relevant reference cell. `y` in fragment [G] or the C snippet above has the type `int`, and only ever evaluates to `int` values.
+(C has explicit-style mutable variables, too, which it calls *pointers*. But simple variables in C are already mutable, in the implicit style.)
+
+When dealing with explicit-style mutation, there's a difference between the types and values of `ycell` and `!ycell` (or in Scheme, `(unbox ycell)`). The former has the type `int ref`: the variable `ycell` is assigned a reference cell that contains an `int`. The latter has the type `int`, and has whatever value is now stored in the relevant reference cell. In an implicit-style framework though, we only have the resources to refer to the contents of the relevant reference cell. `y` in fragment [G] or the C snippet above has the type `int`, and only ever evaluates to `int` values.
##Controlling order##
##Referential opacity##
-In addition to order-sensitivity, when you're dealing with mutable variables you also give up a property that computer scientists call "referential transparency." It's not obvious whether they mean exactly the same by that as philosophers and linguists do, or only something approximately the same. What they do mean is a kind of substitution principle, illustrated here:
+In addition to order-sensitivity, when you're dealing with mutable variables you also give up a property that computer scientists call "referential transparency." It's not obvious whether they mean exactly the same by that as philosophers and linguists do, or only something approximately the same.
+
+The core idea to referential transparency is that when the same value is supplied to a context, the whole should always evaluate the same way. Mutation makes it possible to violate this. Consider:
+
+ let ycell = ref 1
+ in let f x = x + !ycell
+ in let first = f 1 (* first is assigned the value 2 *)
+ in ycell := 2; let second = f 1 (* second is assigned the value 3 *)
+ in first = second;; (* not true! *)
+
+Notice that the two invocations of `f 1` yield different results, even though the same value is being supplied as an argument to the same function.
+
+Similarly, functions like these:
+
+ let f cell = !cell;;
+
+ let g cell = cell := !cell + 1; !cell;;
+
+may return different results each time they're invoked, even if they're always supplied one and the same reference cell as argument.
+
+Computer scientists also associate referential transparency with a kind of substitution principle, illustrated here:
let x = 1
in (x, x)
(* then creates a *new* ref 1 cell and returns *its* contents *)
+
+
##How to implement explicit-style mutable variables##
We'll think about how to implement explicit-style mutation first. We suppose that we add some new syntactic forms to a language, let's call them `newref`, `deref`, and `setref`. And now we want to expand the semantics for the language so as to interpret these new forms.
> \[[expression]]<sub>g s</sub> = (value, s')
-With that kind of framework, we can interpret `newref`, `deref`, and `setref` as follows.
+For expressions we already know how to interpret, expect `s'` to just be `s`.
+An exception is complex expressions like `let var = expr1 in expr2`. Part of
+interpreting this will be to interpret the sub-expression `expr1`, and we have
+to allow that in doing that, the store may have already been updated. We want
+to use that possibly updated store when interpreting `expr2`. Like this:
+
+ let rec eval expression g s =
+ match expression with
+ ...
+ | Let (c, expr1, expr2) ->
+ let (value, s') = eval expr1 g s
+ (* s' may be different from s *)
+ (* now we evaluate expr2 in a new environment where c has been associated
+ with the result of evaluating expr1 in the current environment *)
+ eval expr2 ((c, value) :: g) s'
+ ...
+
+Similarly:
+
+ ...
+ | Addition (expr1, expr2) ->
+ let (value1, s') = eval expr1 g s
+ in let (value2, s'') = eval expr2 g s'
+ in (value1 + value2, s'')
+ ...
+
+Let's consider how to interpet our new syntactic forms `newref`, `deref`, and `setref`:
+
1. \[[newref starting_val]] should allocate a new reference cell in the store and insert `starting_val` into that cell. It should return some "key" or "index" or "pointer" to the newly created reference cell, so that we can do things like:
let rec eval expression g s =
match expression with
...
- | Newref expr ->
+ | Newref (expr) ->
let (starting_val, s') = eval expr g s
(* note that s' may be different from s, if expr itself contained any mutation operations *)
(* now we want to retrieve the next free index in s' *)
in (Index new_index, s'')
...
-2. When `expr` evaluates to a `store_index`, then `deref expr` should evaluate to whatever value is at that index in the current store. (If `expr` evaluates to a value of another type, `deref expr` is undefined.) In this operation, we don't change the store at all; we're just reading from it. So we'll return the same store back unchanged.
+2. When `expr` evaluates to a `store_index`, then `deref expr` should evaluate to whatever value is at that index in the current store. (If `expr` evaluates to a value of another type, `deref expr` is undefined.) In this operation, we don't change the store at all; we're just reading from it. So we'll return the same store back unchanged (assuming it wasn't changed during the evaluation of `expr`).
let rec eval expression g s =
match expression with
...
- | Deref expr ->
+ | Deref (expr) ->
let (Index n, s') = eval expr g s
(* note that s' may be different from s, if expr itself contained any mutation operations *)
in (List.nth s' n, s')
let rec eval expression g s =
match expression with
...
- | Setref expr1 expr2
+ | Setref (expr1, expr2) ->
let (Index n, s') = eval expr1 g s
- (* note that s' may be different from s, if expr itself contained any mutation operations *)
+ (* note that s' may be different from s, if expr1 itself contained any mutation operations *)
in let (new_value, s'') = eval expr2 g s'
(* now we create a list which is just like s'' except it has new_value in index n *)
in let rec replace_nth lst m =
...
+
+
+
##How to implement implicit-style mutable variables##
With implicit-style mutation, we don't have new syntactic forms like `newref` and `deref`. Instead, we just treat ordinary variables as being mutable. You could if you wanted to have some variables be mutable and others not; perhaps the first sort are written in Greek and the second in Latin. But we will suppose all variables in our language are mutable.
To handle implicit-style mutation, we'll need to re-implement the way we interpret expressions like `x` and `let x = expr1 in expr2`. We will also have just one new syntactic form, `change x to expr1 then expr2`.
-Here's how to implement these. We'll suppose that our assignment function is list of pairs, as in [week6](/reader_monad_for_variable_binding).
+Here's how to implement these. We'll suppose that our assignment function is list of pairs, as in [week7](/reader_monad_for_variable_binding).
let rec eval expression g s =
match expression with
in let value = List.nth s index
in (value, s)
- | Let (c : char) expr1 expr2 ->
+ | Let ((c : char), expr1, expr2) ->
let (starting_val, s') = eval expr1 g s
(* get next free index in s' *)
in let new_index = List.length s'
(* evaluate expr2 using a new assignment function and store *)
in eval expr2 ((c, new_index) :: g) s''
- | Change (c : char) expr1 expr2 ->
+ | Change ((c : char), expr1, expr2) ->
let (new_value, s') = eval expr1 g s
(* lookup which index is associated with Var c *)
in let index = List.assoc c g
We call this a State monad. It's a lot like the Reader monad, except that with the Reader monad, we could only read from the environment. We did have the possibility of interpreting sub-expressions inside a "shifted" environment, but as you'll see, that corresponds to the "shadowing" behavior described before, not to the mutation behavior that we're trying to implement now.
-With a State monad, we call our book-keeping apparatus a "state" or "store" instead of an evironment, and this time we are able to both read from it and write to it. To keep things simple, we'll work here with the simplest possible kind of store, which only holds a single value. One could also have stores that were composed of a list of values, of a length that could expand or shrink, or even more complex structures.
+With a State monad, we call our book-keeping apparatus a "state" or "store" instead of an environment, and this time we are able to both read from it and write to it. To keep things simple, we'll work here with the simplest possible kind of store, which only holds a single value. One could also have stores that were composed of a list of values, of a length that could expand or shrink, or even more complex structures.
Here's the implementation of the State monad, together with an implementation of the Reader monad for comparison:
This passes through the current store unaltered, and also returns a copy of the store as its value. We can use this operation like this:
- some_existing_state_monad >>= fun _ -> get_state >>= (fun cur_state -> ...)
+ some_existing_state_monad_box >>= fun _ -> get_state >>= (fun cur_store -> ...)
-The `fun _ ->` part here discards the value wrapped by `some_existing_state_monad`. We're only going to pass through, unaltered, whatever *store* is generated by that monadic value. We also wrap that store as *our own value*, which can be retrieved by further operations in the `... >>= ...` chain, such as `(fun cur_state -> ...)`.
+The `fun _ ->` part here discards the value wrapped by `some_existing_state_monad_box`. We're only going to pass through, unaltered, whatever *store* is generated by that monadic box. We also wrap that store as *our own value*, which can be retrieved by further operations in the `... >>= ...` chain, such as `(fun cur_store -> ...)`.
The other operation for the State monad will be to update the existing store to a new one. This operation looks like this:
- let set_state (value : int) : dummy state =
- fun s -> (dummy, value);;
+ let set_state (new_store : int) : dummy state =
+ fun s -> (dummy, new_store);;
If we want to stick this in a `... >>= ...` chain, we'll need to prefix it with `fun _ ->` too, like this:
- some_existing_state_monad >>= fun _ -> set_state 100 >>= ...
+ some_existing_state_monad_box >>= fun _ -> set_state 100 >>= ...
-In this usage, we don't care what value is wrapped by `some_existing_state_monad`. We don't even care what store it generates, since we're going to replace that store with our own new store. A more complex kind of `set_state` operation might insert not just some constant value as the new store, but rather the result of applying some function to the existing store. For example, we might want to increment the current store. Here's how we could do that:
+In this usage, we don't care what value is wrapped by `some_existing_state_monad_box`. We don't even care what store it generates, since we're going to replace that store with our own new store. A more complex kind of `set_state` operation might insert not just some constant value as the new store, but rather the result of applying some function to the existing store. For example, we might want to increment the current store. Here's how we could do that:
- some_existing_state_monad >>= fun _ -> get_state >>= (fun cur_state -> set_state (cur_state + 1) >>= ...
+ some_existing_state_monad_box >>= fun _ -> get_state >>= (fun cur_store -> set_state (cur_store + 1) >>= ...
-We can of course define more complex functions that perform the `get_state >>= (fun cur_state -> set_state (cur_state + 1)` as a single operation.
+We can of course define more complex functions that perform the `get_state >>= (fun cur_store -> set_state (cur_store + 1)` as a single operation.
-In general, a State monadic **value** (type `'a state`, what appears at the start of a `... >>= ... >>= ...` chain) is an operation that accepts some starting store as input---where the store might be simple as it is here, or much more complex---and returns a value plus a possibly modified store. This can be thought of as an encoding of some operation on a store serving as a box wrapped around a value.
+In general, a State monadic **box** (type `'a state`, what appears at the start of a `... >>= ... >>= ...` chain) is an operation that accepts some starting store as input---where the store might be simple as it is here, or much more complex---and returns a value plus a possibly modified store. This can be thought of as a static encoding of some computation on a store, which encoding is used as a box wrapped around a value of type `'a`. (And also it's a burrito.)
-State monadic **operations** (type `'a -> 'b state`, what appears anywhere in the middle or end of a `... >>= ... >>= ...` chain) are operations that generate new State monadic values, based on what value was wrapped by the preceding elements in the `... >>= ... >>= ...` chain. The computations on a store that these encode (which their values may or may not be sensitive to) will be chained in the order given by their position in the `... >>= ... >>= ...` chain. That is, the computation encoded by the first element in the chain will accept a starting store s0 as input, and will return (a value and) a new store s1 as output, the next computation will get s1 as input and will return s2 as output, the next computation will get s2 as input, ... and so on.
+State monadic **operations** (type `'a -> 'b state`, what appears anywhere in the middle or end of a `... >>= ... >>= ...` chain) are operations that generate new State monad boxes, based on what value was wrapped by the preceding elements in the `... >>= ... >>= ...` chain. The computations on a store that these encode (which their values may or may not be sensitive to) will be chained in the order given by their position in the `... >>= ... >>= ...` chain. That is, the computation encoded by the first element in the chain will accept a starting store s0 as input, and will return (a value and) a new store s1 as output, the next computation will get s1 as input and will return s2 as output, the next computation will get s2 as input, ... and so on.
To get the whole process started, the complex computation so defined will need to be given a starting store. So we'd need to do something like this:
- let computation = some_state_monadic_value >>= operation >>= operation
+ let computation = some_state_monadic_box >>= operation >>= operation
in computation initial_store;;
Notice: h, p have same value (1), but f (h, p) and f (h, h) differ
+Fine and Pryor on "coordinated contents" (see, e.g., [Hyper-Evaluativity](http://www.jimpryor.net/research/papers/Hyper-Evaluativity.txt))
+
+
##Five grades of mutation involvement##
-- FIXME --
We use the `None`/`Some factorial` option type here just as a way to ensure that the contents of `fact_cell` are of the same type both at the start and the end of the block.
+##Offsite Reading##
+
+* [[!wikipedia Declarative programming]]
+* [[!wikipedia Functional programming]]
+* [[!wikipedia Purely functional]]
+* [[!wikipedia Side effect (computer science) desc="Side effects"]]
+* [[!wikipedia Referential transparency (computer science)]]
+* [[!wikipedia Imperative programming]]
+* [[!wikipedia Reference (computer science) desc="References"]]
+* [[!wikipedia Pointer (computing) desc="Pointers"]]
+* [Pointers in OCaml](http://caml.inria.fr/resources/doc/guides/pointers.html)
+
<!--
-Fine and Pryor on "coordinated contents" (see, e.g., [Hyper-Evaluativity](http://www.jimpryor.net/research/papers/Hyper-Evaluativity.txt))
+# General issues about variables and scope in programming languages #
+
+* [[!wikipedia Variable (programming) desc="Variables"]]
+* [[!wikipedia Free variables and bound variables]]
+* [[!wikipedia Variable shadowing]]
+* [[!wikipedia Name binding]]
+* [[!wikipedia Name resolution]]
+* [[!wikipedia Parameter (computer science) desc="Function parameters"]]
+* [[!wikipedia Scope (programming) desc="Variable scope"]]
+* [[!wikipedia Closure (computer science) desc="Closures"]]
+
-->