week9 tweak

[lambda.git] / week9.mdwn

diff --git a/week9.mdwn b/week9.mdwn
index b2fbb3a..2b80483 100644 (file)
--- a/week9.mdwn
+++ b/week9.mdwn
@@ -291,7 +291,11 @@ Now we're going to relativize our interpretations not only to the assignment fun
 
 >      \[[expression]]<sub>g s</sub> = (value, s')
 
 
 >      \[[expression]]<sub>g s</sub> = (value, s')
 

-For expressions we already know how to interpret, `s'` will usually just be `s`. One 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:
+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 rec eval expression g s =
                match expression with


@@ -304,6 +308,15 @@ For expressions we already know how to interpret, `s'` will usually just be `s`.
                        eval expr2 ((c, value) :: g) s'
                ...
 
                        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`:
 
 
 Let's consider how to interpet our new syntactic forms `newref`, `deref`, and `setref`:
 
 


@@ -332,7 +345,7 @@ Let's consider how to interpet our new syntactic forms `newref`, `deref`, and `s
                                in (Index new_index, 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
 
                let rec eval expression g s =
                        match expression with