Re: Ada vs Fortran for scientific applications

[Brooks Moses <bmoses-nospam@cits1.stanford.edu>]

Dmitry A. Kazakov wrote:
> On Tue, 23 May 2006 17:09:03 GMT, Dick Hendrickson wrote:
>>Fortran 2003 has polymorphic variables which might make it
>>easier to write a complete set of units and operators.  That
>>probably would lose some compile time checking.
> 
> That depends on which kind of polymorphism it is. Ada provides three forms
> of:
> 
> 1. generics (like C++ templates) and overloading
> 2. tagged types (like C++ classes)
> 3. discriminated types.
[...]
> Then there is a problem of constraint propagation. Developing a matrix
> library you surely would like to have dimensioned vectors and matrices all
> constrained in a "coherent" way rather than on per-element basis. I doubt
> that either 1. or 2. would be able to do it in an easy way.

For what it's worth, the OpenFOAM CFD program that I've been working 
with (written in C++) has dimensioned variables, with dimension 
checking.  It's all run-time, though; they've only got one type of 
"dimensioned variable" (a class with the variable value and a 
length-seven array containing the exponents of the dimensions) and then 
assignments and arithmetic operators check to make sure the dimensions 
agree.

What they've done for their array library is to define a dimensioned 
array by adding dimension information to an array of undimensioned 
scalars, rather than constructing it directly as an array of dimensioned 
individual numbers.

In practice, it seems to work quite well, and I suspect the run-time 
checking is generally of negligible cost since it's happening only once 
per array operation.  (Of course, in code where it does happen once per 
scalar operation, it's rather more expensive.)

- Brooks


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