On Wed, 24 Jun 2015, Makarius wrote:
The keyword 'when' may be used instead of 'if', to indicate 'presume'
instead of 'assume' above.
This refers to Isabelle/51a6997b1384.
The 'presume' element is rarely used, and the 'when' eigen-context is mainly
an exercise in orthogonality of the language. There might be useful
applications nonetheless, e.g. see the examples about "suffices-to-show" in
~~/src/HOL/Isar_Examples/Structured_Statements.thy
Now that 'presume' semantics has a separate syntax, the normal ==> is free
to be re-interpreted. So here is another breaking NEWS entry for
Isabelle/804dfdc82835:
* The meaning of 'show' with Pure rule statements has changed: premises
are treated in the sense of 'assume', instead of 'presume'. This means,
a goal like "⋀x. A x ⟹ B x ⟹ C x" can be solved completely as follows:
show "⋀x. A x ⟹ B x ⟹ C x"
or:
show "C x" if "A x" "B x" for x
Rare INCOMPATIBILITY, the old behaviour may be recovered as follows:
show "C x" when "A x" "B x" for x
This means the common beginner-mistake to use Pure rule statements for
show no longer leads to bad surprises. As a consequence, there might be
more ugly proofs emerging than necessary, but better than no proof at all
in the first attempt.
Makarius
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