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

 From: Jens Axel Søgaard (jensaxel at soegaard.net) Date: Tue Jul 24 07:33:55 EDT 2012 Previous message: [racket] Commercial Users of Functional Programming 2012: Call for Participation Next message: [racket] Typed Racket and macros expanding to for*: Messages sorted by: [date] [thread] [subject] [author]

```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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