[pro] Bottom-up, top-down or both ?

[John Q. Splittist]

On Fri, Mar 11, 2011 at 5:18 PM, Didier Verna <didier at lrde.epita.fr> wrote:
>
> It is well known that a functional approach to programming helps
> thinking in a top-down fashion. Idioms made available by first-class
> functions (e.g. mapping and folding) help you write down the general
> ideas first, and worry about the details later. Paul Graham has nice
> examples of that in On Lisp IIRC.

In fact 'On Lisp' makes the opposite point. From the Preface: 'As well
as writing their programs down toward the language, experienced Lisp
programmers build the language up toward their programs. This book
teaches how to program in the bottom-up style for which Lisp is
inherently well-suited.'

During the brief period of my university career when people were
trying to teach me programming, top down ('structured') was the way to
go, not just in Pascal, but in COBOL as well. But that was when
writing a program meant using a pen and a specially printed pad of
paper (with the important columns highlighted), so having one's first
thoughts accepted by a compiler was not an issue. It was also an era
where the usual (or at least assumed) case was a well-specified
outcome e.g. produce this report that takes 3 accounts clerks 1 week
to produce in only one overnight batch job. PG (in 'On Lisp') says
this era is over, and that specifications are now 'extremely complex,
or even open-ended.' He continues: ' The theme of this book is
twofold: that Lisp is a natural language for programs written in the
bottom-up style, and that the bottom-up style is a natural way to
write Lisp programs.'

To answer your questions: both. As, I suspect, for all programmers,
always, everywhere.

JQS