[plt-scheme] htdp section 17.7

From: Felix Klock's PLT scheme proxy (pltscheme at pnkfx.org)
Date: Fri Jul 7 20:54:49 EDT 2006


On Jul 7, 2006, at 8:10 PM, wooks . wrote:

> Apologies if this is sent twice ...... can't remember whether I  
> pressed send the first time.
> Please check my data analysis and examples.

My initial response: the data definitions seem close to something  
that might work.  There are two main points I want to make:

1.) I'm a little worried about the places where you have written, for  
> A multiplication is a list (* left right)
> where
>    left is an expression
>    right is an expression

When you say "a list (* left right)", do you mean that one constructs  
such a thing by typing:
(* E1 E2)
where E1 and E2 are expressions, or by typing:
(list '* E1 E2)
where E1 and E2 are expressions, or something else entirely?

There is a very big difference between these two expressions, and to  
help you remember this difference, I might recommend that you not use  
lists to represent multiplications, but instead use actual  
structures, e.g. of the form:
(make-felixs-mult E1 E2)
where you will have to come up with the structure definition so that  
the appropriate constructor is available for you to use.

 From the way you wrote your data defintions for funcInv and funcDef,  
I'm not convinced you remember the right way to develop data  
definitions for structured data.  Go reread Section 6 of HtDP; look  
at the example data defintions *carefully*.

2.) There is one important case that your examples do not cover: the  
case where funcDef you are passing along has a function invocation  
within its body.

That is, consider a scheme definition like:

(define (f y)
   (+ 2 (g (* 3 y))))

None of your examples have a funcDef that looks like f above.

Unfortunately, its actually quite difficult to come up with an  
example like the above that you could actually use in a test for  
evaluate-with-one-def, because evaluate-with-one-def is specified as  
taking only a single funcDef, and so there is no way to actually  
evaluate an invocation of f with evaluate-with-one-def that could  
correctly yield anything except an error. [[ But this will matter  
when you get to 17.7.4. ]]

So, how else could we include an invocation in our function's body  
for 17.7.3?  Can you come up with an example of a funcDef that has an  
invocation in its body that would not be an erroneous input to  
evaluate-with-one-def?  If so, can you anticipate any reason why  
testing an invocation of such a funcDef might still pose a problem?   
(This is a hard question, and there are different correct answers,  
depending on how you approached section 14.4.)


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