Re: [isabelle] evaluation of expressions



Hi Manuel,

Thanks for the suggestion. I briefly had thought about this, but then dismissed it because Viorel wrote that a "quick way" to evaluate such expressions, and code-unfold is not quick as it is done by the simplifier.

Andreas

On 01/03/2019 21:48, Manuel Eberl wrote:
Hello,

there is one possible solution that Andreas may have missed. Whether
this works for you depends a bit on your application, but at least in
the case you presented, you can simply use "g_S_simp" as a "code_unfold"
rule instead of a "code" rule, and then "value" works on your example:

lemma g_S_simp[code_unfold, simp]: "g (S f) = f"

You can read up on what "code_unfold" does in the code generator manual;
very briefly, if I recall correctly, it just rewrites the input
expression with the unfold rules before attempting to generate code for
it. I often use it to generate code for "non-executable" constructs that
can be rewritten into a more palatable form, or to introduce optimisations.

Manuel

On 01/03/2019 21:25, Andreas Lochbihler wrote:
Dear Viorel,

value "..." tries to generate SML code for the given expression and
evaluate it in the run-time system. In your example, the lemma g_S_simp
is not a code equation, because in a code equation, you can only pattern
match on constructors. This is also what the warning tells you.
Accordingly, the code generator ignores this equation and tries to
generate code for g using its definition. This definition contains SOME
and that's where the well-sortedness error comes from.

So there's no way to get to this behaviour for any of the evaluator. The
reason is that the code generator simply does not allow such
non-constructor pattern, although the symbolic evaluators code_simp and
normalization could deal with such equations.

Andreas


On 01/03/2019 09:54, Viorel Preoteasa wrote:
Hello,


I have a lemma of the following form: "g (S f) = f" and I would like
to use it evaluate for example "g (S Suc) 3" using


value "g (S Suc) 3"


Is it possible to achieve this behavior? Simplification works OK, but
I need also a quick way to calculate this kind of expressions.


Next is the complete example


definition "S f x = f x"


lemma S_simp: "S f = f"

    by (simp add: S_def fun_eq_iff)


definition "g A = (SOME f . A = S f)"


lemma g_S_simp[code,simp]: "g (S f) = f"
    by (simp add: g_def S_simp)

lemma "g (S Suc) 3 = 4"
    by (simp) (*this works OK*)

value "g (S Suc) 3" (*this gives Wellsortedness error*)


Best regards,


Viorel Preoteasa


On 19/02/2019 15.14, Thomas Sewell wrote:

I recall the addition of the signed words, and maybe I can add some
information.


I think that "'a signed word" and "'a word" are equivalent for all
key operations. The signed variant was added as part of an adjustment
of the C-to-Isabelle parser, so that in the resulting code it was
possible to tell whether the variables were designated signed or
unsigned in C. Note that most arithmetic is exactly the same.


Also note that the outer type constructor is word, that is, "32
signed word" is an instance of "'a word" where the length parameter
is "32 signed". To make use of this, I think you want to define some
kind of is_signed class with a class constant of type bool, and
ensure that "32 signed" is signed, and that each numeral type counts
as unsigned. Finally, you can lift that to a query on the outer word
type.


I think that all might work, but I haven't tried any of it.


Cheers,

     Thomas.

------------------------------------------------------------------------
*From:* cl-isabelle-users-bounces at lists.cam.ac.uk
<cl-isabelle-users-bounces at lists.cam.ac.uk> on behalf of Viorel
Preoteasa <viorel.preoteasa at gmail.com>
*Sent:* Tuesday, February 19, 2019 10:40:26 AM
*To:* Thiemann, René
*Cc:* cl-isabelle-users at lists.cam.ac.uk
*Subject:* Re: [isabelle] signed and unsigned words
Dear René,

Thank you for your message.

On 18/02/2019 15.18, Thiemann, René wrote:
Dear Viorel,

it is difficult to test your problem, since the “signed” and
“usigned” types are something that I do neither find in the
distribution nor in the AFP.

The definition of the signed type is here:

https://www.isa-afp.org/browser_info/current/AFP/Word_Lib/Signed_Words.html


and the definition of unsigned is identical.


Hence, I can only blindly guess:

There might be a problem in your definition of “overflow_add” which
uses the generic “to_int”, not a specific implementation.
This will work if “to_int” is defined as a class-constant (class
some_name = fixes to_int :: …), but I’m not sure whether your
approach via “consts to_int” is supported with code-generation.

So, you might try to reformulate the definition of to_int via class
and instantiation.

This was my first attempt, but I did not manage the instantiation  of
class overflow as 'a signed word .

Now I have implemented a different solution. I created a new copy of the
type 'a word:

typedef (overloaded) 'a::len sword = "UNIV::'a word set"
    morphisms sword_to_word word_to_sword by auto

and I lifted all operations from word to sword. This seems to work, but
I am interested to find the best possible solution.

Best regards,

Viorel



Best regards,
René

I need new types of signed and unsigned words with operations (+,
-, ...) identical with the operations on words, but with a new
operation:

overflow_add a b which is defined differently for signed and
unsigned words.

Basically I need the two types to be instantiations of the
following class:

class overflow = plus +
    fixes overflow_add::  "'a ⇒ 'a ⇒ bool"

I tried something inspired from the AFP entry "Finite Machine Word
Library":

type_synonym 'a sword = "'a signed word"
type_synonym 'a uword = "'a usigned word"

consts to_int :: "'a word ⇒ int"

overloading
    to_int_sword ≡ "to_int:: 'a sword ⇒ int"
    to_int_uword ≡ "to_int::'a uword ⇒ int"
begin
    definition "to_int_sword (a:: 'a::len sword) = sint a"
    definition "to_int_uword (a:: 'a::len uword) = uint a"
end

instantiation  word :: (len0)  overflow
begin
    definition "overflow_add a b = (to_int (a::'a word) + to_int b =
to_int (a + b))"
instance ..
end

this seems to work, but I don't know how to get code generation
for overflow_add:

value "overflow_add (-2::4 sword) (-3)"

gives the error:

No code equations for to_int








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