make-environment in PLT Scheme

[Alexander Schmidt]

Hi,

I am using:

DrScheme, version 208.
Language: Textual (MzScheme, includes R5RS).

I have entered the following definition:

(define arctic
(make-environment
(define animal 'polarbaer)))

(define ocean
(make-environment
(define animal 'shark)))

And obtain the following error:

Welcome to DrScheme, version 208.
Language: Textual (MzScheme, includes R5RS).
define: not allowed in an expression context in: (define animal (quote
polarbaer))
>

What is wrong?
My lecture notes state the above definition as an example but I cannot get
it working :-(

Any help is appreciated.

Thanks.

[Cláudio F. Gil]

Alexander Schmidt wrote:
> Hi,
>
> I am using:
>
> DrScheme, version 208.
> Language: Textual (MzScheme, includes R5RS).
>
> I have entered the following definition:
>
> (define arctic
> (make-environment
> (define animal 'polarbaer)))
>
> (define ocean
> (make-environment
> (define animal 'shark)))
>
> And obtain the following error:
>
> Welcome to DrScheme, version 208.
> Language: Textual (MzScheme, includes R5RS).
> define: not allowed in an expression context in: (define animal (quote
> polarbaer))

Some scheme implementations can have "define" implemented with other
semantics but in MzScheme, and as stated in the R5RS, definitions are valid
at top level and at the beginning of a body. This means that "define"
normally could only be used with forms like let other defines, etc that
expect a body.

> What is wrong?
> My lecture notes state the above definition as an example but I cannot get
> it working :-(

In MzScheme there is no "make-environment" also. I don't know what your
letcures are about but you could use other operators. An example that
probably does not do what you want is:

(define artic (let ((environment (null-environment 5)))
(eval '(define animal 'polarbaer)
environment)
environment))

"artic" will be and environment that, besides simple syntactic definitions,
has animal defined as the symbol "polarbaer".

[Alexander Schmidt]

"Cláudio F. Gil" <cf...@esw.inesc-id.pt> wrote in message
news:32836kF...@individual.net...

Does anyone know a Scheme implemenentation where my samplecode might run?
So, one which knows make-environment and can have define implemented with
other semantics?

Thanks.

[Jens Axel Søgaard]

Alexander Schmidt wrote:

>>>And obtain the following error:
>>>
>>>Welcome to DrScheme, version 208.
>>>Language: Textual (MzScheme, includes R5RS).
>>>define: not allowed in an expression context in: (define animal (quote
>>>polarbaer))

Your lecture notes probably define environments before giving the
above examples.

Are the lecture notes online somewhere?

--
Jens Axel Søgaard

[Cláudio F. Gil]

Alexander Schmidt wrote:

> Does anyone know a Scheme implemenentation where my samplecode might run?
> So, one which knows make-environment and can have define implemented with
> other semantics?

Both MIT Scheme and UMB Scheme have the "make-environment". If you want you
can define it in MzScheme with more or less the same semantic:

(define-syntax make-environment
(syntax-rules ()
((_ definition ...)
(let ((environment (scheme-report-environment 5)))
(eval '(begin definition
...)
environment)
environment))))

(define arctic
(make-environment
(define animal 'polarbaer)))

(define ocean
(make-environment
(define animal 'shark)))

The difference between this an MIT Scheme is that "make-environment", in MIT
Scheme, extends the current environment and in MzScheme it always extends a
R5RS environment.