Re: language design implications for variant records in a pascal-like language

torbenm@diku.dk (Torben Ægidius Mogensen)
Tue, 04 Jan 2011 12:05:37 +0100

          From comp.compilers

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[18 earlier articles]
Re: language design implications for variant records in a pascal-like bobduff@shell01.TheWorld.com (Robert A Duff) (2010-12-31)
Re: language design implications for variant records in a pascal-like bobduff@shell01.TheWorld.com (Robert A Duff) (2010-12-31)
Re: language design implications for variant records in a pascal-like fw@deneb.enyo.de (Florian Weimer) (2010-12-31)
Re: language design implications for variant records in a pascal-like noitalmost@cox.net (noitalmost) (2010-12-31)
Re: language design implications for variant records in a pascal-like gah@ugcs.caltech.edu (glen herrmannsfeldt) (2011-01-02)
Re: language design implications for variant records in a pascal-like gene.ressler@gmail.com (Gene) (2011-01-02)
Re: language design implications for variant records in a pascal-like torbenm@diku.dk (2011-01-04)
Re: language design implications for variant records in a pascal-like jm@bourguet.org (Jean-Marc Bourguet) (2011-01-05)
Re: language design implications for variant records in a pascal-like gneuner2@comcast.net (George Neuner) (2011-01-06)
Re: language design implications for variant records in a pascal-like gneuner2@comcast.net (George Neuner) (2011-01-06)
Re: language design implications for variant records in a pascal-like gneuner2@comcast.net (George Neuner) (2011-01-06)
Re: language design implications for variant records in a pascal-like gneuner2@comcast.net (George Neuner) (2011-01-06)
Re: language design implications for variant records in a pascal-like bobduff@shell01.TheWorld.com (Robert A Duff) (2011-01-06)
[31 later articles]
| List of all articles for this month |

From: torbenm@diku.dk (Torben Ægidius Mogensen)
Newsgroups: comp.compilers
Date: Tue, 04 Jan 2011 12:05:37 +0100
Organization: SunSITE.dk - Supporting Open source
References: 10-12-040
Keywords: design, storage
Posted-Date: 04 Jan 2011 11:22:12 EST

noitalmost <noitalmost@cox.net> writes:




> My question is this: Would variant records provide significant value
> to a user of my language (given that classes were implemented)?


You can also reverse the question: Given that you have records and
(tagged) unions (== sum types), will classes provide significant value?


If I had to chose either sum types or classes, my choice will be sum
types -- especially if you have ML-style pattern matching.


For example, syntax trees are much easier to represent using sum types
than using classes.


You can define subtyping without introducing classes:


A record Y is a subtype of a record X if for every field f of type t in
X there is a field f of type t' in Y such that t' is a subtype of t.


A sum type Y is a subtype of a sum type X if for every tag (constructor)
c in Y with type t there is a tag c in X with type t' such that t is a
subtype of t'.


Note the symmetry: Subtypes of a sum have fewer tags and subtypes of a
record have more fields. The empty record can be seen as the top type
in the subtype relation while the empty sum (no possible value) is the
bottom type.


Torben



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