What is Expressiveness in a Computer Language
Joachim Durchholz
jo at durchholz.org
Mon Jun 26 17:31:49 EDT 2006
Darren New schrieb:
> Marshall wrote:
>> Also: has subtyping polymorphism or not, has parametric polymorphism or
>> not.
>
> And covariant or contravariant.
That's actually not a design choice - if you wish to have a sound type
system, all input parameters *must* be contravariant, all output
parameters *must* be covariant, and all in/out parameters must be
novariant. (Eiffel got this one wrong in almost all cases.)
Regards,
Jo
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