[TUHS] The 'usage: ...' message. (Was: On Bloat...)

[Douglas McIlroy <douglas.mcilroy@dartmouth.edu>]

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>> Another non-descriptive style of error message that I admired was that
>> of Berkeley Pascal's syntax diagnostics. When the LR parser could not
>> proceed, it reported where, and automatically provided a sample token
>> that would allow the parsing to progress. I found this uniform
>> convention to be at least as informative as distinct hand-crafted
>> messages, which almost by definition can't foresee every contingency.
>> Alas, this elegant scheme seems not to have inspired imitators.

> The hazard with this approach is that the suggested syntactic correction
> might simply lead the user farther into the weeds

I don't think there's enough experience to justify this claim. Before I
experienced the Berkeley compiler, I would have thought such bad outcomes
were inevitable in any language. Although the compilers' suggestions often
bore little or no relationship to the real correction,  I always found them
informative. In particular, the utterly consistent style assured there was
never an issue of ambiguity or of technical jargon.

The compiler taught me Pascal in an evening. I had scanned the Pascal
Report a couple of years before but had never written a Pascal program.
With no manual at hand, I looked at one program to find out what
mumbo-jumbo had to come first and how to print integers, then wrote the
rest by trial and error. Within a couple of hours  I had a working program
good enough to pass muster in an ACM journal.

An example arose that one might think would lead "into the weeds". The
parser balked before 'or' in a compound Boolean expression like  'a=b and
c=d or x=y'. It couldn't suggest a right paren because no left paren had
been seen. Whatever suggestion it did make (perhaps 'then') was enough to
lead me to insert a remote left paren and teach me that parens are required
around Boolean-valued subexpressions. (I will agree that this lesson might
be less clear to a programming novice, but so might be many conventional
diagnostics, e.g. "no effect".)

Doug

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