(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 > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
