# Re: [isabelle] Type as argument

`With the example, would it be possible to let f be a parameter of a locale
``instead, ie
`
locale A1 =
fixes f :: "nat => 'a itself => bool"
defines "fs (t::nat itself) = (if s = a then True else False)"

`complains about the operand "t::nat itself". Is it asking for "nat" to be a
``parameter?
`
Thanks
On May 16, 2011 3:53pm, Brian Huffman <brianh at cs.pdx.edu> wrote:

2011/5/16 Steve W s.wong.731 at gmail.com>:

> Thanks all. One more question: what is the "itself" keyword for in "'a

> itself"?

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".

- Brian

>

> Thanks

> Steve

>

> 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

>> sides.

>> Usually, one can use "fun" instead. However, even with "fun" you cannot

>> do pattern matching on "int" instead of "nat".

>>

`>> However, you might write "fx (t:: nat itself) = (if x = (0 :: int)
``then 0
`

>> else ...)"

>>

>> René

>> --

>> René Thiemann mailto:rene.thiemann at uibk.ac.at

>> Computational Logic Group

>> http://cl-informatik.uibk.ac.at/~thiemann/

>> Institute of Computer Science phone: +43 512 507-6434

>> University of Innsbruck

>>

>>

>

*This archive was generated by a fusion of
Pipermail (Mailman edition) and
MHonArc.*