[plt-scheme] Re: writing an evaluation function/interpreter; type-case

Use a type-case that has one clause (case) per alternative in the
define-type, and then use 'if' or 'cond' or something like that in the
corresponding case.

Robby

On Thu, Feb 18, 2010 at 7:54 AM, ih <irvin.hwang at gmail.com> wrote:
> Thanks for the response.  I'm not sure what it means to "push the test
> down one level."
>
> On Feb 17, 5:35 pm, Jay McCarthy <jay.mccar... at gmail.com> wrote:
>> On Wed, Feb 17, 2010 at 3:21 PM, ih <irvin.hw... at gmail.com> wrote:
>> > Hi,
>> > I'm trying to follow Ch. 4 of Types and Programming Languages by
>> > Benjamin Pierce and implement the language
>>
>> > t:= true | false | if t then t else t
>> > OR using define-type
>> > (define-type term
>> >  [TmTrue (t (lambda (a) (equal? a #t)))]
>> >  [TmFalse (f (lambda (a) (equal? a #f)))]
>> >  [TmIf (t1 term?) (t2 term?) (t3 term?)])
>>
>> > I was trying to use the forms define-type and type-case from PLAI by
>> > Shriram Krishnamurthi, but type-case doesn't seem to have the same
>> > pattern matching functionality as is used Pierce's ocaml
>> > implementation i.e.  I can't write
>>
>> > (define (eval1 e)
>> >  (type-case term e
>> >             [TmTrue (t) t]
>> >             [TmFalse(f) f]
>> >             [TmIf (TmTrue t2 t3) (eval1 t2)]
>> >             [TmIf (TmFalse t2 t3) (eval1 t3)]))
>>
>> > I was trying to use "match", but it doesn't seem to be defined to work
>> > with the algebraic data type defined with define-type (or perhaps I am
>> > using match incorrectly).
>>
>> > (define (eval1 e)
>> >  (type-case term e
>> >             [TmTrue (t) t]
>> >             [TmFalse(f) f]
>> >             [TmIf (t1 t2 t3) (match t1)
>> > ]
>>
>> >  Can anyone give guidance on the proper way to implement this in
>> > scheme?
>>
>> You can push the test down one level.
>>
>> Or you can use match and define-struct.
>>
>> > Should I be using match with define-type?
>>
>> They do not work together.
>>
>>
>>
>> > _________________________________________________
>> >  For list-related administrative tasks:
>> >  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>>
>> --
>> Jay McCarthy <j... at cs.byu.edu>
>> Assistant Professor / Brigham Young Universityhttp://teammccarthy.org/jay
>>
>> "The glory of God is Intelligence" - D&C 93
>> _________________________________________________
>>   For list-related administrative tasks:
>>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> _________________________________________________
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