Simple Test Coverage: A Macro with Line Numbers and Lifting
Racket’s macro system makes it easy to roll your own low-tech line coverage tool. In this post, I’ll show how, in 15 lines of code, you can implement a simple test-coverage tool. Using this code is simple: put (line-of-interest) on each line that should be covered.
To start the implementation, we put the code in a module and define two sets:
#lang racket (define candidate-lines (set)) (define touched-lines (set))
The first set holds the line numbers where (line-of-interest) is written in the source and the second holds the set of line numbers where (line-of-interest) has been executed.
Each use of (line-of-interest) is going to expand into a call to visited with the line number for the source location of that use of (line-of-interest).
(define (visited line) (unless (set-member? touched-lines line) (set! touched-lines (set-add touched-lines line)) (displayln (sort (set->list (set-subtract candidate-lines touched-lines)) <))))
This function simply checks to see if this line has been executed before and, if not, removes that line number from touched-lines and prints out the current status.
The interesting part of this code is in the definition of line-of-interest itself:
(define-syntax (line-of-interest stx) (with-syntax ([line (syntax-line stx)]) (syntax-local-lift-expression #'(set! candidate-lines (set-add candidate-lines line))) #'(visited line)))
The macro first extracts the line number from stx, which gives the source location for the use of (line-of-interest). This number is then bound to line for use in building later syntax objects. Then the macro calls syntax-local-lift-expression with a syntax object that updates candidate-lines. Expressions passed to syntax-local-lift-expression are lifted to the top-level of the enclosing module making sure that, in this case, each line number is added exactly once without having to execute the code where (line-of-interest) appears. The macro then discards the result of syntax-local-lift-expression and returns a call to the visited function. That’s all there is to it!
I originally used this macro to test some changes to DrRacket. I was working on a set of complex GUI interactions and kept losing track of which ones had been tested and which ones hadn’t. Here’s a simpler program in the same spirit so you can try it out.
#lang racket/gui (define candidate-lines (set)) (define touched-lines (set)) (define (visited line) (unless (set-member? touched-lines line) (set! touched-lines (set-add touched-lines line)) (displayln (sort (set->list (set-subtract candidate-lines touched-lines)) <)))) (define-syntax (line-of-interest stx) (with-syntax ([line (syntax-line stx)]) (syntax-local-lift-expression #'(set! candidate-lines (set-add candidate-lines line))) #'(visited line))) (define f (new frame% [label ""])) (define b1 (new button% [label "1"] [parent f] [callback (λ (a b) (case (random 3) [(0) (line-of-interest) (send b1 set-label "one")] [(1) (line-of-interest) (send b1 set-label "uno")] [(2) (line-of-interest) (send b1 set-label "一")]))])) (define b2 (new button% [label "2"] [parent f] [callback (λ (a b) (case (random 3) [(0) (line-of-interest) (send b2 set-label "two")] [(1) (line-of-interest) (send b2 set-label "dos")] [(2) (line-of-interest) (send b2 set-label "二")]))])) (send f show #t)
