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added a potentially useful macro

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This commit is contained in:
Matthias Felleisen 2012-12-27 16:33:18 -05:00
parent 6243a928fb
commit 01bb5c400a
1 changed files with 30 additions and 5 deletions
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35
collects/2htdp/tests/batch-io3.rkt
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@ -66,7 +66,32 @@ eos
(module+ test
(define (split file n)
(values (drop file n) (take file n))))
(values (drop file n) (take file n)))
(require (for-syntax syntax/parse))
;; (parse-file x:id ([y:id e:expr][(u:id ...) f:expr] ...) body:expr)
;; teases apart x (a list of lines) according to the expressions e, f, ... and successively binds
;; the values in the lines to y, u, ... and eventually evaluates the body ... expressions
;; Restriction: to one body expressions so that this form does not inject errors into *SLs
(define-syntax (parse-file stx)
(syntax-parse stx
[(parse-file file:id ((x:id e:expr) clauses ...) body:expr)
#'(let-values ([(x file) (split-at file (enni e))])
(parse-file file (clauses ...) body))]
[(parse-file file:id (((x:id ...) e:expr) clauses ...) body:expr)
#'(let-values ([(file x ...)
(let-values ([(bundle file) (split-at file (enni e))])
(apply values file (first bundle)))])
(parse-file file (clauses ...) body))]
[(parse-file file:id () body) #'body]))
;; Any -> Exact-nonnegative-integer
;; coercion with error message for parse-file
(define (enni x)
(if (exact-nonnegative-integer? x) x (error 'parse-file pem x)))
(define pem "exact-nonnegative-integer expected for relative file position; given: ~e"))
(module+ test
;; String -> Void
@ -78,15 +103,15 @@ eos
(cond
[(zero? ds#) ""]
[else
(let*-values ([(file sales-records# queries#) (apply values (rest file) (first file))]
[(file sales-records) (split file sales-records#)]
[(file queries) (split file queries#)])
(parse-file file ([(sales-record# queries#) 1.02]
[sales-records sales-record#]
[queries queries#])
(string-append (process-dataset sales-records queries) "\n" (loop (sub1 ds#) file)))])))
;; Dataset -> String
;; process one dataset
(define (process-dataset sales-records queries)
(foldr (λ (query r) (string-append (total-message sales-records (first query)) r)) "" queries))
;; [Listof [List Number String Number]] String -> String
;; compute query result and turn into string
(define (total-message sales-records query)