EWD249 - 75

On what we have achieved.

One of the metaphors in which I find myself thinking about the program structure envisaged regards the program as a necklace, strung from individual pearls.

We have described the program in terms of levels and each level contained "refinements" of entities that were assumed available in higher levels. These refinements were either dynamic refinements (algorithms) or static refinements (data structures) to be understood by an appropriate machine. I use the term "pearl" for such a machine, refinements included.

Our previous program consists of a necklace of six pearls, in order either

COMPFIRST
CLEARFIRST
ISCANNER
COMPPOS
LINER
LONGREP

or

COMPFIRST
CLEARFIRST
ISCANNER
COMPPOS
LINER
SHORTREP

LONGREP and SHORTREP are two different pearls, they explain the same concepts (from the "upper face") into the same concept (of the "lower face"); only the particular refinements differ: they are as alternative programs for the same job and the same machine.

Changing a program will be treated as replacing one or more pearls of the original necklace by one or more other pearls. The pearl is the individual unit from which programs are composed. Making a program (as a member of a class of related programs) is now regarded as a two-stage process: making pearls (more than