Re: Why are Ada compilers difficult to write ?

["J-P. Rosen" <rosen@adalog.fr>]

Le 21/06/2018 à 22:18, Dan'l Miller a écrit :
> Pascal, what makes freezing rules difficult when writing Ada compilers? 
> 1) Is it that the freezing occurs later than what the compiler writer
> would find convenient, forcing the compiler writer to keep clay soft
> when it already seems to be in the kiln?> 2) Is it that freezing occurs prematurely, requiring the compiler
> writer to expend effort enforcing the freezing too early for no good
> reason?> 3) Is it that freezing rules are non-uniform/arbitrary/capricious
> across language constructs?> 4) Something else?
> 5) All of the above?

I'm not Pascal ;-), but I'll answer these...
Please do not assume that language designers are masochists that make
the language unnecessarily complicated for no good reason. There are
good reasons, and these can usually be found in the annotated reference
manual.

For example, if freezing happened at the end of a package (as you
suggest), it would not be possible to declare a variable or a constant
in the same package as its type (the type has to be frozen - i.e. its
representation determined - for an object to be declared). Therefore,
the declaration of an object freezes the type. Then, when you freeze a
type, of course you have to freeze all types of subcomponents. etc...
You end up with 13.14.

You may argue that the way 13.14 is written is not very... pedagogical.
True, but this is really for compiler writers, therefore rigor of
definition is more important than clarity of the statement. The casual
user just needs to rely on the compiler's message telling that some
clause happens too late.

-- 
J-P. Rosen
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