Re: [isabelle] Type as argument
2011/5/16 Steve W <s.wong.731 at gmail.com>:
> Thanks all. One more question: what is the "itself" keyword for in "'a
The "itself" is just a type constructor. It is like what you would get
from the following datatype declaration:
datatype 'a itself = Type (* one constructor with no arguments *)
The syntax "TYPE('a)" is an abbreviation for "Type :: 'a itself".
> 2011/5/13 René Thiemann <rene.thiemann at uibk.ac.at>
>> > Thanks. If I change f to a binary function:
>> > class foo =
>> > fixes f :: "int => 'a itself => int"
>> > and with the following instantiation:
>> > instantiation nat :: foo
>> > begin
>> > definition "f 0 (t::nat itself) = 0"
>> > instance ..
>> > end
>> > I get an error on "0::int". It works if 0 is replaced by a free variable
>> > though. Why's that?
>> Because "definition" cannot be used with pattern matching on left-hand
>> Usually, one can use "fun" instead. However, even with "fun" you cannot
>> do pattern matching on "int" instead of "nat".
>> However, you might write "f x (t:: nat itself) = (if x = (0 :: int) then 0
>> else ...)"
>> René Thiemann mailto:rene.thiemann at uibk.ac.at
>> Computational Logic Group
>> Institute of Computer Science phone: +43 512 507-6434
>> University of Innsbruck
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