[racket-dev] TR: case-> and for/sum:

Apropos the discussion on porting from Racket to Typed Racket.
The following snippet was written in Racket, and now I want
to add types.

The problem is that I have used for/sum and due to the case->
I can't see how to change the for/sum to a for/sum: .

/Jens Axel

#lang typed/racket
(require math)

(: matrix-solve-upper
   (All (A) (case->
             ((Matrix Real)   (Matrix Real)          -> (Matrix Real))
             ((Matrix Real)   (Matrix Real)   (-> A) -> (U A (Matrix Real)))
             ((Matrix Number) (Matrix Number)        -> (Matrix Number))
             ((Matrix Number) (Matrix Number) (-> A) -> (U A (Matrix
Number))))))
(define (matrix-solve-upper U b)
  (define (default-fail)
    (raise-argument-error 'matrix-solve-upper
                          "The upper triangular matrix is not invertible." 0 U))
  ; solve the equation Ux=b
  ; using back substitution
  (case-lambda
    [(U b) (matrix-solve-upper U b default-fail)]
    [(U b fail)
     (define m (matrix-num-rows U))
     (define x (make-vector m 0))
     (for ([i (in-range (- m 1) -1 -1)])
       (define bi (matrix-ref b i 0))
       (define Uii (matrix-ref U i i))
       (when (zero? Uii) (fail))
       (define x.Ui (for/sum ([j (in-range (+ i 1) m)])
                      (* (matrix-ref U i j) (vector-ref x j))))
       (vector-set! x i (/ (- bi x.Ui) Uii)))
     (vector->matrix m 1 x)]))

(define U (matrix [[4 -1 2  3]
                   [0 -2 7 -4]
                   [0  0 6  5]
                   [0  0 0  3]]))
(define b (col-matrix [20 -7 4 6]))
b ; expected
(define x (solve-upper-triangular U b))
(matrix* U x)