327 lines
13 KiB
Scheme
327 lines
13 KiB
Scheme
;****************************************************************************
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; Simple Parsing of input
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;
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; The following simple functions surprisingly often suffice to parse
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; an input stream. They either skip, or build and return tokens,
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; according to inclusion or delimiting semantics. The list of
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; characters to expect, include, or to break at may vary from one
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; invocation of a function to another. This allows the functions to
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; easily parse even context-sensitive languages.
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;
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; EOF is generally frowned on, and thrown up upon if encountered.
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; Exceptions are mentioned specifically. The list of expected characters
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; (characters to skip until, or break-characters) may include an EOF
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; "character", which is to be coded as symbol *eof*
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;
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; The input stream to parse is specified as a PORT, which is usually
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; the last (and optional) argument. It defaults to the current input
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; port if omitted.
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;
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; IMPORT
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; This package relies on a function parser-error, which must be defined
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; by a user of the package. The function has the following signature:
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; parser-error PORT MESSAGE SPECIALISING-MSG*
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; Many procedures of this package call parser-error to report a parsing
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; error. The first argument is a port, which typically points to the
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; offending character or its neighborhood. Most of the Scheme systems
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; let the user query a PORT for the current position. MESSAGE is the
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; description of the error. Other arguments supply more details about
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; the problem.
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; myenv.scm, myenv-bigloo.scm or a similar prelude is assumed.
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; From SRFI-13, string-concatenate-reverse
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; If a particular implementation lacks SRFI-13 support, please
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; include the file srfi-13-local.scm
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;
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; $Id: input-parse.scm,v 1.2 2004/11/09 14:11:40 sperber Exp $
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;------------------------------------------------------------------------
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; -- procedure+: peek-next-char [PORT]
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; advances to the next character in the PORT and peeks at it.
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; This function is useful when parsing LR(1)-type languages
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; (one-char-read-ahead).
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; The optional argument PORT defaults to the current input port.
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(define-opt (peek-next-char (optional (port (current-input-port))))
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(read-char port)
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(peek-char port))
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;------------------------------------------------------------------------
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; -- procedure+: assert-curr-char CHAR-LIST STRING [PORT]
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; Reads a character from the PORT and looks it up
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; in the CHAR-LIST of expected characters
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; If the read character was found among expected, it is returned
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; Otherwise, the procedure writes a nasty message using STRING
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; as a comment, and quits.
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; The optional argument PORT defaults to the current input port.
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;
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(define-opt (assert-curr-char expected-chars comment
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(optional (port (current-input-port))))
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(let ((c (read-char port)))
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(if (memv c expected-chars) c
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(parser-error port "Wrong character " c
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" (0x" (if (eof-object? c) "*eof*"
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(number->string (char->integer c) 16)) ") "
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comment ". " expected-chars " expected"))))
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; -- procedure+: skip-until CHAR-LIST [PORT]
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; Reads and skips characters from the PORT until one of the break
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; characters is encountered. This break character is returned.
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; The break characters are specified as the CHAR-LIST. This list
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; may include EOF, which is to be coded as a symbol *eof*
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;
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; -- procedure+: skip-until NUMBER [PORT]
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; Skips the specified NUMBER of characters from the PORT and returns #f
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;
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; The optional argument PORT defaults to the current input port.
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(define-opt (skip-until arg (optional (port (current-input-port))) )
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(cond
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((number? arg) ; skip 'arg' characters
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(do ((i arg (dec i)))
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((not (positive? i)) #f)
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(if (eof-object? (read-char port))
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(parser-error port "Unexpected EOF while skipping "
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arg " characters"))))
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(else ; skip until break-chars (=arg)
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(let loop ((c (read-char port)))
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(cond
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((memv c arg) c)
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((eof-object? c)
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(if (memq '*eof* arg) c
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(parser-error port "Unexpected EOF while skipping until " arg)))
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(else (loop (read-char port))))))))
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; -- procedure+: skip-while CHAR-LIST [PORT]
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; Reads characters from the PORT and disregards them,
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; as long as they are mentioned in the CHAR-LIST.
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; The first character (which may be EOF) peeked from the stream
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; that is NOT a member of the CHAR-LIST is returned. This character
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; is left on the stream.
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; The optional argument PORT defaults to the current input port.
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(define-opt (skip-while skip-chars (optional (port (current-input-port))) )
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(do ((c (peek-char port) (peek-char port)))
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((not (memv c skip-chars)) c)
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(read-char port)))
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; whitespace const
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;------------------------------------------------------------------------
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; Stream tokenizers
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; -- procedure+:
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; next-token PREFIX-CHAR-LIST BREAK-CHAR-LIST [COMMENT-STRING] [PORT]
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; skips any number of the prefix characters (members of the
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; PREFIX-CHAR-LIST), if any, and reads the sequence of characters
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; up to (but not including) a break character, one of the
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; BREAK-CHAR-LIST.
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; The string of characters thus read is returned.
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; The break character is left on the input stream
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; The list of break characters may include EOF, which is to be coded as
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; a symbol *eof*. Otherwise, EOF is fatal, generating an error message
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; including a specified COMMENT-STRING (if any)
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;
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; The optional argument PORT defaults to the current input port.
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;
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; Note: since we can't tell offhand how large the token being read is
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; going to be, we make a guess, pre-allocate a string, and grow it by
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; quanta if necessary. The quantum is always the length of the string
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; before it was extended the last time. Thus the algorithm does
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; a Fibonacci-type extension, which has been proven optimal.
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; Note, explicit port specification in read-char, peek-char helps.
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; Procedure: input-parse:init-buffer
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; returns an initial buffer for next-token* procedures.
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; The input-parse:init-buffer may allocate a new buffer per each invocation:
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; (define (input-parse:init-buffer) (make-string 32))
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; Size 32 turns out to be fairly good, on average.
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; That policy is good only when a Scheme system is multi-threaded with
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; preemptive scheduling, or when a Scheme system supports shared substrings.
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; In all the other cases, it's better for input-parse:init-buffer to
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; return the same static buffer. next-token* functions return a copy
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; (a substring) of accumulated data, so the same buffer can be reused.
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; We shouldn't worry about an incoming token being too large:
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; next-token will use another chunk automatically. Still,
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; the best size for the static buffer is to allow most of the tokens to fit in.
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; Using a static buffer _dramatically_ reduces the amount of produced garbage
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; (e.g., during XML parsing).
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(define input-parse:init-buffer
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(let ((buffer (make-string 512)))
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(lambda () buffer)))
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; See a better version below
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(define-opt (next-token-old prefix-skipped-chars break-chars
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(optional (comment "") (port (current-input-port))) )
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(let* ((buffer (input-parse:init-buffer))
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(curr-buf-len (string-length buffer))
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(quantum curr-buf-len))
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(let loop ((i 0) (c (skip-while prefix-skipped-chars port)))
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(cond
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((memv c break-chars) (substring buffer 0 i))
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((eof-object? c)
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(if (memq '*eof* break-chars)
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(substring buffer 0 i) ; was EOF expected?
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(parser-error port "EOF while reading a token " comment)))
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(else
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(if (>= i curr-buf-len) ; make space for i-th char in buffer
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(begin ; -> grow the buffer by the quantum
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(set! buffer (string-append buffer (make-string quantum)))
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(set! quantum curr-buf-len)
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(set! curr-buf-len (string-length buffer))))
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(string-set! buffer i c)
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(read-char port) ; move to the next char
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(loop (inc i) (peek-char port))
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)))))
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; A better version of next-token, which accumulates the characters
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; in chunks, and later on reverse-concatenates them, using
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; SRFI-13 if available.
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; The overhead of copying characters is only 100% (or even smaller: bulk
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; string copying might be well-optimised), compared to the (hypothetical)
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; circumstance if we had known the size of the token beforehand.
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; For small tokens, the code performs just as above. For large
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; tokens, we expect an improvement. Note, the code also has no
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; assignments.
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; See next-token-comp.scm
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(define-opt (next-token prefix-skipped-chars break-chars
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(optional (comment "") (port (current-input-port))) )
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(let outer ((buffer (input-parse:init-buffer)) (filled-buffer-l '())
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(c (skip-while prefix-skipped-chars port)))
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(let ((curr-buf-len (string-length buffer)))
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(let loop ((i 0) (c c))
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(cond
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((memv c break-chars)
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(if (null? filled-buffer-l) (substring buffer 0 i)
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(string-concatenate-reverse filled-buffer-l buffer i)))
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((eof-object? c)
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(if (memq '*eof* break-chars) ; was EOF expected?
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(if (null? filled-buffer-l) (substring buffer 0 i)
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(string-concatenate-reverse filled-buffer-l buffer i))
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(parser-error port "EOF while reading a token " comment)))
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((>= i curr-buf-len)
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(outer (make-string curr-buf-len)
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(cons buffer filled-buffer-l) c))
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(else
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(string-set! buffer i c)
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(read-char port) ; move to the next char
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(loop (inc i) (peek-char port))))))))
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; -- procedure+: next-token-of INC-CHARSET [PORT]
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; Reads characters from the PORT that belong to the list of characters
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; INC-CHARSET. The reading stops at the first character which is not
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; a member of the set. This character is left on the stream.
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; All the read characters are returned in a string.
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;
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; -- procedure+: next-token-of PRED [PORT]
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; Reads characters from the PORT for which PRED (a procedure of one
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; argument) returns non-#f. The reading stops at the first character
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; for which PRED returns #f. That character is left on the stream.
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; All the results of evaluating of PRED up to #f are returned in a
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; string.
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;
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; PRED is a procedure that takes one argument (a character
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; or the EOF object) and returns a character or #f. The returned
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; character does not have to be the same as the input argument
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; to the PRED. For example,
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; (next-token-of (lambda (c)
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; (cond ((eof-object? c) #f)
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; ((char-alphabetic? c) (char-downcase c))
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; (else #f))))
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; will try to read an alphabetic token from the current
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; input port, and return it in lower case.
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;
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; The optional argument PORT defaults to the current input port.
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;
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; This procedure is similar to next-token but only it implements
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; an inclusion rather than delimiting semantics.
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(define-opt (next-token-of incl-list/pred
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(optional (port (current-input-port))) )
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(let* ((buffer (input-parse:init-buffer))
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(curr-buf-len (string-length buffer)))
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(if (procedure? incl-list/pred)
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(let outer ((buffer buffer) (filled-buffer-l '()))
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(let loop ((i 0))
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(if (>= i curr-buf-len) ; make sure we have space
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(outer (make-string curr-buf-len) (cons buffer filled-buffer-l))
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(let ((c (incl-list/pred (peek-char port))))
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(if c
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(begin
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(string-set! buffer i c)
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(read-char port) ; move to the next char
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(loop (inc i)))
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; incl-list/pred decided it had had enough
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(if (null? filled-buffer-l) (substring buffer 0 i)
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(string-concatenate-reverse filled-buffer-l buffer i)))))))
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; incl-list/pred is a list of allowed characters
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(let outer ((buffer buffer) (filled-buffer-l '()))
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(let loop ((i 0))
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(if (>= i curr-buf-len) ; make sure we have space
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(outer (make-string curr-buf-len) (cons buffer filled-buffer-l))
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(let ((c (peek-char port)))
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(cond
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((not (memv c incl-list/pred))
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(if (null? filled-buffer-l) (substring buffer 0 i)
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(string-concatenate-reverse filled-buffer-l buffer i)))
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(else
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(string-set! buffer i c)
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(read-char port) ; move to the next char
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(loop (inc i))))))))
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)))
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; -- procedure+: read-text-line [PORT]
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; Reads one line of text from the PORT, and returns it as a string.
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; A line is a (possibly empty) sequence of characters terminated
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; by CR, CRLF or LF (or even the end of file).
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; The terminating character (or CRLF combination) is removed from
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; the input stream. The terminating character(s) is not a part
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; of the return string either.
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; If EOF is encountered before any character is read, the return
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; value is EOF.
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;
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; The optional argument PORT defaults to the current input port.
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(define *read-line-breaks* (list char-newline char-return '*eof*))
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(define-opt (read-text-line (optional (port (current-input-port))) )
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(if (eof-object? (peek-char port)) (peek-char port)
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(let* ((line
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(next-token '() *read-line-breaks*
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"reading a line" port))
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(c (read-char port))) ; must be either \n or \r or EOF
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(and (eqv? c char-return) (eqv? (peek-char port) #\newline)
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(read-char port)) ; skip \n that follows \r
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line)))
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; -- procedure+: read-string N [PORT]
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; Reads N characters from the PORT, and returns them in a string.
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; If EOF is encountered before N characters are read, a shorter string
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; will be returned.
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; If N is not positive, an empty string will be returned.
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; The optional argument PORT defaults to the current input port.
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(define-opt (read-string n (optional (port (current-input-port))) )
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(if (not (positive? n)) ""
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(let ((buffer (make-string n)))
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(let loop ((i 0) (c (read-char port)))
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(if (eof-object? c) (substring buffer 0 i)
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(let ((i1 (inc i)))
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(string-set! buffer i c)
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(if (= i1 n) buffer
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(loop i1 (read-char port)))))))))
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