Hello,
I have been interested in working the exercises in the CTM book. I did
a brief, general reading of the book about three months ago and generated
some traffic on this mailing list with questions. At this time I care to
resume my efforts of understanding Mozart/Oz. I have contacted Peter Van
Roy, and he said that he welcomes solutions of CTM exercises to be posted to
the mailing list. I am looking forward to comments regarding the
correctness of my interpretations of answers to the exercises. Here are two
problems from CTM chapter 2:
*
1.* *Free and bound identifiers*. Consider the following statement:
*
proc {P X}
if X>0 then {P X-1} end
end
*
Is the second occurence of the identifier P free or bound? Justify your
answer. Hint: this is easy to answer if you first translate to kernel
syntax.
*
The kernel syntax for the above code is as follows:
P = proc {$ X}
local P in
local T in
T = (X > 0)
if T then P = proc {$ X-1} end
end
end
**
The second occurence of the parameter P is bound. The P in the inner
statement is declared by a local statement that is inserted into
the code upon translation to the kernel syntax. Because the P is declared as
a local variable, it is added to the environment and its value
is placed on the store, making it executable.**
2.* *Contextual environment*. Section 2.4 explains how a procedure call is
executed. Consider the following procedure MulByN:
*
declare MulByN in
N = 3
proc {MulByN X ?Y}
Y=N*X
end
*
together with the call {MulByN A B}. Assume that the environment at the call
contains {A->10, B->x1}. When the procedure body is
executed, the mapping N->3 is added to the environment. Why is this a
necessary step? In particular, would not N->3 already exist
somewhere in the environment at the call? Would not this be enough to insure
that the identifier N already maps to 3? Give an example
where N does not exist in the environment at the call. Then give an example
where N does exist there, but is bound to a different value
than 3.
*
Before the procedure application the kernel semantics looks like:
**
( [ ( { MulByN A B}, {A->a, B->x1, N->n, MulyByN->m} ) ], { m = ( proc {$ X
Y} Y=N*X ) end, {N->n} ), a=10, b, n=3 } )
**
After the procedure application the kernel semantics looks like:
**
( [ ( Y = N*X, {X->a, Y->x1, N->n} ) ], { m = ( proc {$ A B} Y=N*X ) , {
N->n } ), a = 10, b, n = 3 } )
**
Although the mapping N->n exists in the environment and the mapping n=3
exists in the store before the procedure application, it is
necessary to have mapping N->n in the procedure value to identify the
contextual environment such that the external reference can be
mapped to the formal parameters of MulByN. In another case, there could be
other mappings in the environment that do not factor into
the procedure application, and we do not want to get them confused with
MulyByN's formal parameters.
Here is an example where N does not exist in the environment at the time of
the call:
**
declare MulByN in
proc {MulByN X ?Y}
Y=N*X
end
**
Before the procedure application the kernel semantics looks like:
**( [ ( { MulByN A B}, {A->a, B->x1, MulByN->m} ) ], { m = ( proc {$ X Y }
Y=N*X ) end }, a=10, b } )
*
*
The procedure application is suspended because the contextual environment
for N is missing.
Here is an example where N exists at the time of the call, but is bound to a
different value than 3.
**
declare MulByN in
N = 3
proc {MulByN X ?Y}
Y=N*X
end
local N in
N = 5
{MulByN A B}
end
**
Before the procedure application, the kernel semantics looks like:
**( [ ( { MulByN A B}, {A->a, B->x1, N->n', MulyByN->m} ) ], { m = ( proc {$
X Y} Y=N*X ) end, {N->n} ), a=10, b, n=3, n'=5 } )*
*
After the procedure application the kernel semantics looks like:
**( [ ( Y = N*X, {X->a, Y->x1, N->n} ) ], { m = ( proc {$ A B} Y=N*X ) , {
N->n } ), a=10, b, n=3, n'=5 } )
*
*This is the other case mentioned above. The variable N is mapped to the
value n', yet the procedure application changes this mapping *
*back to the value n because this was the value mapped to by the contextual
environment.*
*
Sincerely,*
*
Craig Ugoretz*
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