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8.4 Scheme I/O
Different Scheme's may have quite different implementations of file i/o functions so in this section we will describe
the basic functions in Festival SIOD regarding i/o.
Simple printing to the screen may be achieved with the function print which prints the given s-expression to the
screen. The printed form is preceded by a new line. This is often useful for debugging but isn't really powerful
enough for much else.
Files may be opened and closed and referred to file descriptors in a direct analogy to C's stdio library. The SIOD
functions fopen and fclose work in the exactly the same way as their equivalently named partners in C.
The format command follows the command of the same name in Emacs and a number of other Lisps. C
programmers can think of it as fprintf. format takes a file descriptor, format string and arguments to print. The
file description may be a file descriptor as returned by the Scheme function fopen, it may also be t which means
the output will be directed as standard out (cf. printf). A third possibility is nil which will cause the output to
printed to a string which is returned (cf. sprintf).
The format string closely follows the format strings in ANSI C, but it is not the same. Specifically the directives
currently supported are, %%, %d, %x, %s, %f, %g and %c. All modifiers for these are also supported. In addition %l is
provided for printing of Scheme objects as objects.
For example
(format t "%03d %3.4f %s %l %l %l\n" 23 23 "abc" "abc" '(a b d) utt1)
will produce
023 23.0000 abc "abc" (a b d) #
on standard output.
When large lisp expressions are printed they are difficult to read because of the parentheses. The function pprintf
prints an expression to a file description (or t for standard out). It prints so the s-expression is nicely lined up and
indented. This is often called pretty printing in Lisps.
For reading input from terminal or file, there is currently no equivalent to scanf. Items may only be read as Scheme
expressions. The command
(load FILENAME t)
will load all s-expressions in FILENAME and return them, unevaluated as a list. Without the third argument the
load function will load and evaluate each s-expression in the file.
To read individual s-expressions use readfp. For example
(let ((fd (fopen trainfile "r"))
(entry)
(count 0))
(while (not (equal? (set! entry (readfp fd)) (eof-val)))
(if (string-equal (car entry) "home")
(set! count (+ 1 count))))
(fclose fd))