[racket] idiomatic way to loop over 1-255 in TR?

Probably fixnum has the same problem.

Even Non-Typed-Racket has a under the hood optimization for loops with
fixnums, but it doesn't detect the corner case (in-range 1073741824).

Just an idea: Is it possible to change (in-range 1073741824)
automatically to (in-range-inclusive 1073741823) and allowd additional
optimizations? Nevertheless, (in-range 256) is probably more usual.

Gustavo


On Wed, Feb 18, 2015 at 4:59 PM, Neil Toronto <neil.toronto at gmail.com> wrote:
> TR isn't really good about finding precise types for range sequence
> elements. The macro-expanded code is too complicated.
>
> When I write a loop with an index that needs to have a specific, narrow
> type, I either use an assertion, as in
>
>   (for/list : (Listof Byte) ([b  (in-range 256)])
>     (assert b byte?))
>
> or loop using the next wider type and always compare the loop index with a
> value that has the narrow type:
>
>   (reverse
>    (let loop : (Listof Byte) ([b : Index  0]
>                               [bs : (Listof Byte)  empty])
>      (cond [(<= b 255)  (loop (+ b 1) (cons b bs))]
>            [else  bs])))
>
> The second method is about 10% faster.
>
> Neil ⊥
>
>
> On 02/18/2015 02:38 PM, John Clements wrote:
>>
>> I want to write this:
>>
>> (for/list ([b (in-range 256)])
>>    b)
>>
>> ... and get back a list of bytes.
>>
>> That is, I want TR to see that the number 256 is one greater than the
>> largest byte, and therefore that b must be of type byte.  I see that
>> this dips way too close to undecidability in the general case, so I
>> would instead perhaps expect to see something like an 'in-bytes-range'
>> form that is known to produce only bytes.
>>
>> I looked for such a thing, but didn't find it.  Naturally, I could
>> simply jam a run-time check in at the top of the body, something like
>> 'ensure-bytes', but it seems unnecessary.
>>
>> Am I missing something obvious?
>>
>> Thanks!
>>
>> John
>>
>>
>>
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