Path: utzoo!attcan!uunet!cs.utexas.edu!asuvax!ncar!ico!ism780c!news
From: news@ism780c.isc.com (News system)
Newsgroups: comp.lang.misc
Subject: Re: Typing on units
Message-ID: <40346@ism780c.isc.com>
Date: 16 Mar 90 04:04:14 GMT
References: <39941@ism780c.isc.com> <883@enea.se>
Reply-To: marv@ism780.UUCP (Marvin Rubenstein)
Organization: Interactive Systems Corp., Santa Monica CA
Lines: 64

In article <883@enea.se> sommar@enea.se (Erland Sommarskog) writes:
>Marvin Rubenstein (marv@ism780.UUCP) writes:
  I (Marv Rubinstein) talked about using units for an aid in error
   detection.

[Erland Sommarskog responded]
:The program below contains the same error and does not compile.
:I have edited the error messages to stay below 80 columns.
:
:   PROCEDURE Type_test IS
:
:       TYPE Meter             IS NEW Float;
:       TYPE Seconds           IS NEW Float;
:       TYPE Meter_per_seconds IS NEW Float;
:
:       x : Meter   := 2.2;
:       s : Seconds := 3.3;
:       v : Meter_per_seconds;
:
:       FUNCTION "/"(x : Meter; s : Seconds) RETURN Meter_per_seconds IS
:       BEGIN
:          RETURN Meter_per_seconds(x) / Meter_per_seconds(s);
:       END "/";
:
:   BEGIN
:      v := x / s;
:      v := x + s;
:   --------^A
:   ------------^B
:   --A:error: RM 3.3: base type of expression must be meter_per_seconds, line 5
:   --B:error: RM 3.3: base type of expression must be meter_per_seconds, line 5
:   END Type_test;
:
:In order to get division to work we had to define a function for it.
:In fact we have to define a function of a similar kind for every operation
:we want to make.


The language I had in mind was more like this.

     units
       m              -- mass
       l              -- length
       t              -- time
       f = m*l/(t*t)  -- force
     end units
     data
       velocity :real, unit l/t;
       distance :real, unit l;
       time     :real, unit t;
       area     :real, unit l*l;
     end data
     velocity = area/(time*distance); -- this is ok
     area     = distance*distance;    -- so is this
     time     =velocity/distance;     -- not good
     area     =distance+distance;     -- bad

The actual language that I was refering to also allowed conversion
constants (e.g. for feet to meters) to appear in the units declarations.
Thus, conversions were inserted automatically by the compiler.  What you are
suggesting would, in my opinion, make the programming of a large scientific
computation almost impossible.

     Marv Rubinstein