An `Rd.T`

(or ``reader'') is a character input stream. The basic
operation on a reader is `GetChar`

, which returns the source
character at the ``current position'' and advances the current
position by one. Some readers are ``seekable'', which means that
they also allow setting the current position anywhere in the
source. For example, readers from random access files are
seekable; readers from terminals and sequential files are not.
\index{character input stream}
\index{input stream}
\index{stream!input}
\index{reader}

Some readers are ``intermittent'', which means that the source of the reader trickles in rather than being available to the implementation all at once. For example, the input stream from an interactive terminal is intermittent. An intermittent reader is never seekable.

Abstractly, a reader `rd`

consists of

len(rd)These values are not necessarily directly represented in the data fields of a reader object. In particular, for an intermittent reader,the number of source characterssrc(rd)a sequence of lengthlen(rd)+1cur(rd)an integer in the range[0..len(rd)]avail(rd)an integer in the range[cur(rd)..len(rd)+1]closed(rd)a booleanseekable(rd)a booleanintermittent(rd)a boolean

`len(rd)`

may be unknown to the implementation. But in
principle the values determine the state of the reader.
The sequence `src(rd)`

is zero-based: `src(rd)[i]`

is valid for `i`

from 0 to `len(rd`

). The first `len(rd)`

elements of `src`

are the
characters that are the source of the reader. The final element is
a special value `eof`

used to represent end-of-file. The value
`eof`

is not a character.

The value of `cur(rd)`

is the index in `src(rd)`

of the next
character to be returned by `GetChar`

, unless `cur(rd) = len(rd)`

,
in which case a call to `GetChar`

will raise the exception
`EndOfFile`

.

The value of `avail(rd)`

is important for intermittent readers: the
elements whose indexes in `src(rd)`

are in the range
`[cur(rd)..avail(rd)-1]`

are available to the implementation and
can be read by clients without blocking. If the client tries to
read further, the implementation will block waiting for the other
characters. If `rd`

is not intermittent, then `avail(rd)`

is equal
to `len(rd)+1`

. If `rd`

is intermittent, then `avail(rd)`

can
increase asynchronously, although the procedures in this interface
are atomic with respect to such increases.

The definitions above encompass readers with infinite sources. If
`rd`

is such a reader, then `len(rd)`

and `len(rd)+1`

are both
infinity, and there is no final `eof`

value.

Every reader is a monitor; that is, it contains an internal lock
that is acquired and held for each operation in this interface, so
that concurrent operations will appear atomic. For faster,
unmonitored access, see the `UnsafeRd`

interface.

If you are implementing a long-lived reader class, such as a pipe or TCP stream, the index of the reader may eventually overflow, causing the program to crash with a bounds fault. We recommend that you provide an operation to reset the reader index, which the client can call periodically.

INTERFACESince there are many classes of readers, there are many ways that a reader can break---for example, the connection to a terminal can be broken, the disk can signal a read error, etc. All problems of this sort are reported by raising the exceptionRd ; IMPORT AtomList; FROM Thread IMPORT Alerted; TYPE T <: ROOT; EXCEPTION EndOfFile; Failure(AtomList.T);

`Failure`

. The
documentation of a reader class should specify what failures the
class can raise and how they are encoded in the argument to
`Failure`

.
Illegal operations cause a checked runtime error.

PROCEDURE GetChar(rd: T): CHAR RAISES {EndOfFile, Failure, Alerted};

Return the next character from`rd`

. More precisely, this is equivalent to the following, in which`res`

is a local variable of type`CHAR`

:

IF closed(rd) THENCause checked runtime errorEND;Block untilavail(rd) > cur(rd); IF cur(rd) = len(rd) THEN RAISE EndOfFile ELSE res := src(rd)[cur(rd)]; INC(cur(rd)); RETURN res ENDPROCEDURE GetWideChar(rd: T): WIDECHAR RAISES {EndOfFile, Failure, Alerted};Many operations on a reader can wait indefinitely. For example,Return the next wide character from`rd`

. Two 8-bit bytes are read from`rd`

and concatenated in little-endian order to form a 16-bit character. That is, the first byte read will be the low-order 8 bits of the result and the second byte will be the high-order 8 bits.`GetChar`

can wait if the user is not typing. In general these waits are alertable, so each procedure that might wait includes`Thread.Alerted`

in its`RAISES`

clause.PROCEDURE EOF(rd: T): BOOLEAN RAISES {Failure, Alerted};Return`TRUE`

iff`rd`

is at end-of-file. More precisely, this is equivalent to:

IF closed(rd) THENNotice that on an intermittent reader,Cause checked runtime errorEND;Block untilavail(rd) > cur(rd); RETURN cur(rd) = len(rd)

`EOF`

can block. For example, if there are no characters buffered in a terminal reader,`EOF`

must wait until the user types one before it can determine whether he typed the special key signalling end-of-file. If you are using`EOF`

in an interactive input loop, the right sequence of operations is: \begin{enumerate} \item prompt the user; \item call`EOF`

, which probably waits on user input; \item presuming that`EOF`

returned`FALSE`

, read the user's input. \end{enumerate}PROCEDURE UnGetChar(rd: T) RAISES {};``Push back'' the last character read from`rd`

, so that the next call to`GetChar`

will read it again. More precisely, this is equivalent to the following:

IF closed(rd) THENexcept there is a special rule:Cause checked runtime errorEND; IF cur(rd) > 0 THEN DEC(cur(rd)) END`UnGetChar(rd)`

is guaranteed to work only if`GetChar(rd)`

was the last operation on`rd`

. Thus`UnGetChar`

cannot be called twice in a row, or after`Seek`

or`EOF`

. If this rule is violated, the implementation is allowed (but not required) to cause a checked runtime error.PROCEDURE CharsReady(rd: T): CARDINAL RAISES {Failure};Return some number of characters that can be read without indefinite waiting. The ``end of file marker'' counts as one character for this purpose, so`CharsReady`

will return 1, not 0, if`EOF(rd)`

is true. More precisely, this is equivalent to the following:

IF closed(rd) THENWarning:Cause checked runtime errorEND; IF avail(rd) = cur(rd) THEN RETURN 0 ELSE RETURNsome number in the range[1~..~avail(rd) - cur(rd)]END;

`CharsReady`

can return a result less than`avail(rd) - cur(rd)`

; also, more characters might trickle in just as`CharsReady`

returns. So the code to flush buffered input without blocking requires a loop:

LOOP n := Rd.CharsReady(rd); IF n = 0 THEN EXIT END; FOR i := 1 TO n DO EVAL Rd.GetChar(rd) END END;PROCEDURE GetSub(rd: T; VAR (*OUT*) str: ARRAY OF CHAR) : CARDINAL RAISES {Failure, Alerted};Read from`rd`

into`str`

until`rd`

is exhausted or`str`

is filled. More precisely, this is equivalent to the following, in which`i`

is a local variable:

i := 0; WHILE i # NUMBER(str) AND NOT EOF(rd) DO str[i] := GetChar(rd); INC(i) END; RETURN iPROCEDURE GetWideSub(rd: T; VAR (*OUT*) str: ARRAY OF WIDECHAR) : CARDINAL RAISES {Failure, Alerted};Read from`rd`

into`str`

until`rd`

is exhausted or`str`

is filled. More precisely, this is equivalent to the following, in which`i`

is a local variable:

i := 0; WHILE i # NUMBER(str) AND NOT EOF(rd) DO str[i] := GetWideChar(rd); INC(i) END; RETURN iPROCEDURE GetSubLine(rd: T; VAR (*OUT*) str: ARRAY OF CHAR) : CARDINAL RAISES {Failure, Alerted};Read from`rd`

into`str`

until a newline is read,`rd`

is exhausted, or`str`

is filled. More precisely, this is equivalent to the following, in which`i`

is a local variable:

i := 0; WHILE i # NUMBER(str) AND (i = 0 OR str[i-1] # '\n') AND NOT EOF(rd) DO str[i] := GetChar(rd); INC(i) END; RETURN iPROCEDURE GetWideSubLine(rd: T; VAR (*OUT*) str: ARRAY OF WIDECHAR) : CARDINAL RAISES {Failure, Alerted};Note thatRead from`rd`

into`str`

until a newline is read,`rd`

is exhausted, or`str`

is filled.`GetLine`

strips the terminating line break, while`GetSubLine`

does not.PROCEDURE GetText(rd: T; len: CARDINAL): TEXT RAISES {Failure, Alerted};Read from`rd`

until it is exhausted or`len`

characters have been read, and return the result as a`TEXT`

. More precisely, this is equivalent to the following, in which`i`

and`res`

are local variables:

res := ""; i := 0; WHILE i # len AND NOT EOF(rd) DO res := res & Text.FromChar(GetChar(rd)); INC(i) END; RETURN resPROCEDURE GetWideText(rd: T; len: CARDINAL): TEXT RAISES {Failure, Alerted};Read from`rd`

until it is exhausted or`len`

wide characters have been read, and return the result as a`TEXT`

. More precisely, this is equivalent to the following, in which`i`

and`res`

are local variables:

res := ""; i := 0; WHILE i # len AND NOT EOF(rd) DO res := res & Text.FromWideChar(GetChar(rd)); INC(i) END; RETURN resPROCEDURE GetLine(rd: T): TEXT RAISES {EndOfFile, Failure, Alerted};If`EOF(rd)`

then raise`EndOfFile`

. Otherwise, read characters until a line break is read or`rd`

is exhausted, and return the result as a`TEXT`

---but discard the line break if it is present. A line break is either {\tt "\n"} or {\tt "\r\n"} More precisely, this is equivalent to the following, in which`ch`

and`res`

are local variables:

IF EOF(rd) THEN RAISE EndOfFile END; res := ""; ch := '\000'; (* any char but newline| WHILE ch # '\n' AND NOT EOF(rd) DO | ch := GetChar(rd); | IF ch = '\n' THEN | IF NOT Text.Empty(res) AND | Text.GetChar(res, Text.Length(res)-1) = '\r' THEN | res := Text.Sub(res, 0, Text.Length(res)-1) | END | ELSE | res := res & Text.FromChar(ch) | END | RETURN res *) PROCEDURE GetWideLine(rd: T): TEXT RAISES {EndOfFile, Failure, Alerted};If`EOF(rd)`

then raise`EndOfFile`

. Otherwise, read wide characters until a line break is read or`rd`

is exhausted, and return the result as a`TEXT`

---but discard the line break if it is present. A line break is either {\tt "\n"} or {\tt "\r\n"}.PROCEDURE Seek(rd: T; n: CARDINAL) RAISES {Failure, Alerted};This is equivalent to:

IF closed(rd) OR NOT seekable(rd) THENCause checked runtime errorEND; cur(rd) := MIN(n, len(rd))PROCEDURE Close(rd: T) RAISES {Failure, Alerted};Release any resources associated with`rd`

and set`closed(rd) := TRUE`

. The documentation of a procedure that creates a reader should specify what resources are released when the reader is closed. This leaves`rd`

closed even if it raises an exception, and is a no-op if`rd`

is closed.PROCEDURE Index(rd: T): CARDINAL RAISES {};This is equivalent to:

IF closed(rd) THENCause checked runtime errorEND; RETURN cur(rd)PROCEDURE Length(rd: T): INTEGER RAISES {Failure, Alerted};This is equivalent to:

IF closed(rd) THENIfCause checked runtime errorEND; RETURN len(rd)`len(rd)`

is unknown to the implementation of an intermittent reader,`Length(rd)`

returns -1.PROCEDURE Intermittent(rd: T): BOOLEAN RAISES {}; PROCEDURE Seekable(rd: T): BOOLEAN RAISES {}; PROCEDURE Closed(rd: T): BOOLEAN RAISES {};Return`intermittent(rd)`

,`seekable(rd)`

, and`closed(rd)`

, respectively. These can be applied to closed readers.END Rd.