From 8851b9a8232b479f166c711beae3cc6a665b047c Mon Sep 17 00:00:00 2001
From: Jim Pryor
Date: Fri, 26 Nov 2010 07:52:12 -0500
Subject: [PATCH] tweak calc improvements
Signed-off-by: Jim Pryor
---
advanced_topics/calculator_improvements.mdwn | 41 ++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)
diff --git a/advanced_topics/calculator_improvements.mdwn b/advanced_topics/calculator_improvements.mdwn
index 541e6958..7576b22e 100644
--- a/advanced_topics/calculator_improvements.mdwn
+++ b/advanced_topics/calculator_improvements.mdwn
@@ -661,6 +661,45 @@ The complete code is available [here](/code/calculator/calc6.ml).
##Adding Aliasing and Passing by Reference##
+Next we'll add aliasing as described at the end of [[week9]]. We'll also add the ability to pass (implicit) reference cells as arguments to a function, which lets changes made within the function body to be effective in the outside environment. When we discussed this in [[week9]], we proposed a different syntactic form for the function values that get called in this way. Instead of:
+
+ let f = lambda (y) -> ...
+ ...
+ in f x
+
+one would write:
+
+ let f = lambda (alias y) -> ...
+ ...
+ in f x
+
+Real programming languages that have this ability, such as C++, do something analagous. Here the function is declared so that *all* of its applications are expected to alias the supplied argument. You can always work around that in a particular case, though, like this:
+
+ let f = lambda (alias y) -> ...
+ ...
+ in let y = x ; creates new (implicit) reference cell with x's value
+ in f y
+
+In our present framework, it will be easier to do things differently. We will
+introduce a new syntactic forms at the location where a function value is
+applied, rather than in the function's declaration. So we will say instead:
+
+ Let ('f',
+ Lambda ('y', ...),
+ ...
+ Apply(Variable 'f', Variable 'x')...)
+
+for the familiar, passing-by-value behavior, and:
+
+ Let ('f',
+ Lambda ('y', ...),
+ ...
+ Applyalias(Variable 'f', 'x')...)
+
+for the proposed new, passing-by-reference behavior. (Besides being easier to implement here, this strategy also has the advantage of more closely aligning with the formal system Jim discusses in his "Hyper-evaluativity" paper.) Note that the second parameter to the `Applyalias` form is just `'x'`, not `Variable 'x'`. This is because (1) only variables are acceptable there, not arbitrary expressions, and (2) we don't need at that point to compute the variable's present value.
+
+Here is our expanded language:
+
type term =
Intconstant of int
| Multiplication of (term * term)
@@ -679,6 +718,8 @@ The complete code is available [here](/code/calculator/calc6.ml).
| Applyalias of (term * char)
;;
+The definitions of `index`, `bound_value`, `assignment`, `expressed_value`, and `store` can remain as they were in the implementation of implicit-style mutation. Here are the changes to our evaluation function:
+
let rec eval (t : term) (g : assignment) (s : store) = match t with
...
| Alias (var_to_bind, orig_var, t3) ->
--
2.11.0