[racket] Typed Racket and macros expanding to for*:

(I had just considered writing a matrix library in TR too.) 

I don't see any other way than use a #:when clause that checks i. -- Matthias





On Jul 24, 2012, at 7:33 AM, Jens Axel Søgaard wrote:

> Hi All,
> 
> I am writing a comprehension for*/matrix: that collects
> numbers and puts them in an mxn matrix.
> 
> A simple example:
> 
> (for*/matrix: : Number 2 3 ([i (in-range 6)]) i)
> ; => '#(0 1 2
> ;          3 4 5)
> 
> In this post the representation is simplified to a vector.
> 
> An example more:
> 
> (for*/matrix: : Number 2 3
>  ([i (in-range 2)]
>   [j (in-range 3)])
>  (+ (* 10 i) j))
> ; => '# (   0  1   2
> ;           10 11 12)
> 
> The two examples above work fine with the code below.
> 
> However I'd like the comprehension to automatically when m*n
> numbers have been generated. That is, I'd like
> 
>    (for*/matrix: : Number 2 3 ([i (in-naturals)]) i)
> 
> to produce #(0 1 2 3 4 5).
> 
> The current expansion generates an index out of range error in vector-set!.
> In for/matrix I solved the problem by adding a dummy sequence
> before the for:-clauses provided by the macro user.
> 
>         (for: ([dummy (in-range (* m n))] for:-clause ...)
>           (define x (let () . defs+exprs))
> 
> This works because for: evaluates the sequences in parallel.
> This also implies I can't use the same trick for for*: which
> evaluates the sequences in a nested fashion.
> 
> The question is how to modify the expansion below
> to  automatically stop after m*n elements have been
> generated?
> 
> /Jens Axel
> 
> 
> #lang typed/racket
> 
> (define-syntax (for*/matrix: stx)
>  (syntax-case stx ()
>    [(_ : type m-expr n-expr (for:-clause ...) . defs+exprs)
>     (syntax/loc stx
>       (let ()
>         (define: m   : Index           m-expr)
>         (define: n   : Index           n-expr)
>         (define: m*n : Index           (assert (* m n) index?))
>         (define: v   : (Vectorof type) (make-vector m*n 0))
>         (define: i   : Index           0)
>         (for*: (for:-clause ...)
>           (define x (let () . defs+exprs))
>           (vector-set! v i x)
>           (set! i (assert (+ i 1) index?)))
>         v))]))
> 
> (for*/matrix: : Number 2 3
>  ([i (in-range 2)]
>   [j (in-range 3)])
>  (+ (* 10 i) j))
> ; => '#(  0  1  2
> ;        10 11 12)
> 
> (for*/matrix: : Number 2 3 ([i (in-range 6)]) i)
> ; => '#(0 1 2
> ;       3 4 5)
> 
> (for*/matrix: : Number 2 3 ([i (in-naturals)]) i)
> ; vector-set!: index out of range
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