Best way to do input with racket?

Question

What's the best way to read input from stdin in racket?

In particular I'd like something like cin from c++ or scanf from c where I specify the types of things I want read and they are returned.

Answer 1

read-line is easy. To be portable across Unix and Windows, additional option is required.

(read-line (current-input-port) 'any)

Return and linefeed characters are detected after the conversions that are automatically performed when reading a file in text mode. For example, reading a file in text mode on Windows automatically changes return-linefeed combinations to a linefeed. Thus, when a file is opened in text mode, 'linefeed is usually the appropriate read-line mode.

So, 'any is required to be portable when the input port is not a file (standard input).

Test program:

#lang racket
(let loop ()
    (display "Input: ")
    (define a (read-line (current-input-port) 'any))
    (printf "input: ~a, length: ~a, last character: ~a\n"
        a
        (string-length a)
        (char->integer (string-ref a (- (string-length a) 1))))
    (loop))

In Windows, replace (read-line (current-input-port) 'any) with (read-line) and see what happens.

Answer 2

You can do pretty much everything you want to... at the low level, I would suggest (read-line) and (read-bytes). For higher-level processing (as e.g. scanf does), I would suggest a regexp-match on the input. For instance

(regexp-match #px" *([0-9]+)" (current-input-port))

Answer 3

I'd use the read procedure for the general case. If the data type to be read is known beforehand, use read-char, read-string, read-bytes.

Also, take a look at this implemenation for reading formatted input - a scanf in Scheme.

Answer 4

Here's a basis for line-by-line processing in Racket Scheme. It doesn't split an input line into multiple words, or do typed input, but this seems like a good place to put it.

(define (get)
  (read-line (current-input-port)))

(define (put . vs)
  (for-each display vs)
  (displayln ""))

(define (sed fn)
  (let ((line (get)))
    (if (not (eof-object? line))
      (begin
        (fn line)
        (sed fn))
      'true)))

(sed (lambda (line)
  (put "Hello, " line)))

Here's one that does split input, also encodes CSV for good measure.

(define (get)
  (read-line (current-input-port)))

(define split string-split)
(define sep ",")
(define enc identity)

(define (enc-csv s)
  (string-append "\"" (string-replace s "\"" "\"\"") "\""))
(define enc enc-csv)

(define (put . vs)
  (displayln (string-join (map enc vs) sep)))

(define (sed fn)
  (let ((line (get)))
    (if (not (eof-object? line))
      (begin
        (fn line)
        (sed fn))
      'true)))

(sed (lambda (line)
  (apply put (split line))))

This works in Racket. I'm not sure how much of it is specific to Racket. It doesn't seem to work in Chicken or Guile.