(r5rs.info.gz) Input

Info Catalog

(r5rs.info.gz) Ports

(r5rs.info.gz) Input and output

(r5rs.info.gz) Output

 
 6.6.2 Input
 -----------
 
  
 
 
 
 
 
  -- library procedure: read
  -- library procedure: read port
      `Read' converts external representations of Scheme objects into the
      objects themselves.  That is, it is a parser for the nonterminal
      <datum> (see sections  External representation and 
      Pairs and lists).  `Read' returns the next object parsable from
      the given input PORT, updating PORT to point to the first
      character past the end of the external representation of the
      object.
 
      If an end of file is encountered in the input before any
      characters are found that can begin an object, then an end of file
      object is returned.   The port remains open, and further attempts
      to read will also return an end of file object.  If an end of file
      is encountered after the beginning of an object's external
      representation, but the external representation is incomplete and
      therefore not parsable, an error is signalled.
 
      The PORT argument may be omitted, in which case it defaults to the
      value returned by `current-input-port'.  It is an error to read
      from a closed port.
 
  -- procedure: read-char
  -- procedure: read-char port
      Returns the next character available from the input PORT, updating
      the PORT to point to the following character.  If no more
      characters are available, an end of file object is returned.  PORT
      may be omitted, in which case it defaults to the value returned by
      `current-input-port'.
 
 
  -- procedure: peek-char
  -- procedure: peek-char port
      Returns the next character available from the input PORT,
      _without_ updating the PORT to point to the following character.
      If no more characters are available, an end of file object is
      returned.  PORT may be omitted, in which case it defaults to the
      value returned by `current-input-port'.
 
           _Note:_ The value returned by a call to `peek-char' is the
           same as the value that would have been returned by a call to
           `read-char' with the same PORT.  The only difference is that
           the very next call to `read-char' or `peek-char' on that PORT
           will return the value returned by the preceding call to
           `peek-char'.  In particular, a call to `peek-char' on an
           interactive port will hang waiting for input whenever a call
           to `read-char' would have hung.
 
 
  -- procedure: eof-object? obj
      Returns #t if OBJ is an end of file object, otherwise returns #f.
      The precise set of end of file objects will vary among
      implementations, but in any case no end of file object will ever
      be an object that can be read in using `read'.
 
 
  -- procedure: char-ready?
  -- procedure: char-ready? port
      Returns #t if a character is ready on the input PORT and returns
      #f otherwise.  If `char-ready' returns #t then the next
      `read-char' operation on the given PORT is guaranteed not to hang.
      If the PORT is at end of file then `char-ready?' returns #t.
      PORT may be omitted, in which case it defaults to the value
      returned by `current-input-port'.
 
           _Rationale:_ `Char-ready?' exists to make it possible for a
           program to accept characters from interactive ports without
           getting stuck waiting for input.  Any input editors
           associated with such ports must ensure that characters whose
           existence has been asserted by `char-ready?' cannot be rubbed
           out.  If `char-ready?' were to return #f at end of file, a
           port at end of file would be indistinguishable from an
           interactive port that has no ready characters.
 
 

Info Catalog

(r5rs.info.gz) Ports

(r5rs.info.gz) Input and output

(r5rs.info.gz) Output