Re: Life cycle costs for FORTRAN+Ada vs. Ada

[world!srctran@uunet.uu.net (Gregory Aharonian)]

    There are some problems migrating from Fortran to Ada for scientific
software.  The first is that Fortran allows you to do things with arrays
that is impossible to recreate very well in any other language.  With
Fortran, you cuold pass parts of any dimensional array to a subroutines,
which can look at it like an array of an arbitrary dimensions.  While
this has been put to great use in Fortran (BLAS,LINPACK,LAPACK), it requires
a complete rewrite for strongly typed languages.
     Second, most Fortran science applications use complex arithmetic, whose
implementation in Fortran compilers has been optimized and verified to death.
In Ada (as well as C++) you have to create a complex type and implement all
of the complex libraries yourself, which is not a numericall trivial thing to
do.  I think that there are a few such libraries being offered, but not to
the degree of quality that Ada allows.

     Above 40,000 lines of code, I would suggest that you choose a language
and use it from the outset.  Conversion tools will provide little economic
advantage for second stage translation (and more than likely the guys who
wrote the stuff will not be very excited about translating it to Ada, and
I would not suggest letting anyone else translate their code).

     Actually, teaching a good subset science-version of Ada probably will
not be that difficult, or unpleasant to the scientists, especially if you
turn off dependency-recompiles, which is more aggravating than learning a
new language.

     You might want to contact some of the DoD scientists with major 
Fortran codes (like the Lowtran guys) and ask them why they haven't
converted to Ada (as they still have the freedom to choose).

Greg Aharonian
Source Translation & Optimization