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Re: [Chicken-users] Question about procedure type annotations

From: megane
Subject: Re: [Chicken-users] Question about procedure type annotations
Date: Sat, 27 Oct 2012 19:35:33 +0300
User-agent: mu4e 0.9.9-dev4; emacs 24.1.1

Felix writes:

> From: megane <address@hidden>
> Subject: [Chicken-users] Question about procedure type annotations
> Date: Wed, 24 Oct 2012 18:54:09 +0300
>> Hi!
>> Below is an example that fails.
>> Is this expected behavior? It feels a bit redundant to use 'assume' for
>> the parameter 'a' as the type for the procedure has already been
>> declared.
>> (: foo (fixnum -> undefined))
>> (define (foo a)
>>   (cond-expand
>>    (compiling
>>     (compiler-typecase a
>>       (fixnum #t))
>>     (else #t)))
>>   (print a))
>> ;; Error: in toplevel procedure `foo':
>> ;;   (test1.scm:2) no clause applies in `compiler-typecase' for expression 
>> of type `*':
>> ;;     fixnum
> Nothing prevents me from doing
>   (foo "123")
> The procedure can not expect to be always called with correctly typed
> arguments, from any possible call-site. The flow-analysis is
> intraprocedural and goes not beyond procedure boundaries. You can use
> the "-strict-types" option to declare that "foo" will indeed be called
> with correctly typed arguments - your example should work with that
> option, but calling it with other argument types will result in
> undefined behaviour at run-time.

Ah, of course.
> "assume" overrides whatever type-information is available from
> flow-analysis (as does "the"). 
> One direction would be to generate type-checks at entry to a
> procedure with declared argument types, and I'm currently
> working on something related. This would have a run-time cost,
> but the arguments would than be known to be correct. I'm
> not sure yet what is best.

That's really interesting! Would this be in the core or some kind of an

Am I correct that this would not cause much run-time overhead in simple
cases, as the types of the arguments would be checked anyway at some
point if they are used?

For example in: (define (foo a b) (+ a b)) the `+' would normally check
the types. But if you check the types (e.g. float?) first and then
assume them to be of correct type the `+' would be specialized to a call
that does no type checks.

You could currently do some kind of a procedure defining macro that
defines two versions, one that just does the type-checks and calls the
other. The second version contains the actual body and the arguments are
`assumed' to be of correct type. And, if the types are known at the
call-site it would be specialized to call the second one directly.

This would give a bit more fine grained solution than `-strict-types',
but you would then have two versions of each procedure.

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