forked from xach/dpans
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathconcept-reader-algorithm.tex
225 lines (195 loc) · 8.62 KB
/
concept-reader-algorithm.tex
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
%-*- Mode: TeX -*-
%% Reader Algorithm
%% 22.1.1 5
This section describes the algorithm used by the \term{Lisp reader}
to parse \term{objects} from an \term{input} \term{character} \term{stream},
including how the \term{Lisp reader} processes \term{macro characters}.
When dealing with \term{tokens}, the reader's basic function is to distinguish
representations of \term{symbols} from those of \term{numbers}.
%%Barmar didn't like the double negatives:
% When a \term{token} is
% accumulated, it is assumed to be a \term{number} unless it does
% not satisfy the Syntax for Numbers listed in \figref\SyntaxForNumericTokens.
When a \term{token} is accumulated, it is assumed to represent a \term{number} if it
satisfies the syntax for numbers listed in \figref\SyntaxForNumericTokens.
%%Ditto:
% If it is not a \term{number}, it is then assumed to be a potential
% number unless it does not satisfy the rules governing the syntax for a
% \term{potential number}.
If it does not represent a \term{number},
it is then assumed to be a \term{potential number}
if it satisfies the rules governing the syntax for a \term{potential number}.
If a valid \term{token} is neither a representation of a \term{number}
nor a \term{potential number},
it represents a \term{symbol}.
The algorithm performed by the \term{Lisp reader} is as follows:
%% 22.1.1 6
\beginlist
\item{1.}
If at end of file, end-of-file processing is performed as specified
in \funref{read}.
Otherwise,
one \term{character}, \param{x}, is read from the \term{input} \term{stream}, and
dispatched according to the \term{syntax type} of \param{x} to one
of steps 2 to 7.
%% 22.1.1 7
\item{2.}
If \param{x} is an \term{invalid} \term{character},
an error \oftype{reader-error} is signaled.
%% 22.1.1 8
\item{3.}
If \param{x} is a \term{whitespace}\meaning{2} \term{character},
then it is discarded and step 1 is re-entered.
%% 22.1.1 9
\item{4.}
If \param{x} is a \term{terminating} or \term{non-terminating} \term{macro character}
then its associated \term{reader macro function} is called with two \term{arguments},
the \term{input} \term{stream} and \param{x}.
%% 22.1.1 10
The \term{reader macro function} may read \term{characters}
from the \term{input} \term{stream};
if it does, it will see those \term{characters} following the \term{macro character}.
The \term{Lisp reader} may be invoked recursively from the \term{reader macro function}.
%% 22.1.5 16
The \term{reader macro function} must not have any side effects other than on the
\term{input} \term{stream};
because of backtracking and restarting of the \funref{read} operation,
front ends to the \term{Lisp reader} (\eg ``editors'' and ``rubout handlers'')
may cause the \term{reader macro function} to be called repeatedly during the
reading of a single \term{expression} in which \param{x} only appears once.
%% 22.1.1 11
The \term{reader macro function} may return zero values or one value.
If one value is returned,
then that value is returned as the result of the read operation;
the algorithm is done.
If zero values are returned, then step 1 is re-entered.
%% 22.1.1 12
\item{5.}
If \param{x} is a \term{single escape} \term{character}
then the next \term{character}, \param{y}, is read, or an error \oftype{end-of-file}
is signaled if at the end of file.
\param{y} is treated as if it is a \term{constituent}
whose only \term{constituent trait} is \term{alphabetic}\meaning{2}.
\param{y} is used to begin a \term{token}, and step 8 is entered.
%% 22.1.1 13
\item{6.}
If \param{x} is a \term{multiple escape} \term{character}
then a \term{token} (initially
containing no \term{characters}) is begun and step 9 is entered.
%% 22.1.1 14
\item{7.}
If \param{x} is a \term{constituent} \term{character}, then it begins a \term{token}.
After the \term{token} is read in, it will be interpreted
either as a \Lisp\ \term{object} or as being of invalid syntax.
If the \term{token} represents an \term{object},
that \term{object} is returned as the result of the read operation.
If the \term{token} is of invalid syntax, an error is signaled.
% If \param{x} is a \term{lowercase} \term{character},
% it is replaced with the corresponding \term{uppercase} \term{character}.
%% Tentatively replaced with the following to satisfy Sandra:
If \param{x} is a \term{character} with \term{case},
it might be replaced with the corresponding \term{character} of the opposite \term{case},
depending on the \term{readtable case} of the \term{current readtable},
as outlined in \secref\ReadtableCaseReadEffect.
\param{X} is used to begin a \term{token}, and step 8 is entered.
%% 22.1.1 15
%% 22.1.1 16
%% 22.1.1 17
\item{8.}
At this point a \term{token} is being accumulated, and an even number
of \term{multiple escape} \term{characters} have been encountered.
If at end of file, step 10 is entered.
Otherwise, a \term{character}, \param{y}, is read, and
one of the following actions is performed according to its \term{syntax type}:
\beginlist
\itemitem{\bull}
If \param{y} is a \term{constituent} or \term{non-terminating} \term{macro character}:
\beginlist
\itemitem{--}
% If \param{y} is a \term{lowercase} \term{character}, it is replaced with the
% corresponding \term{uppercase} \term{character}.
%% Tentatively replaced with the following to satisfy Sandra:
If \param{y} is a \term{character} with \term{case},
it might be replaced with the corresponding \term{character} of the opposite \term{case},
depending on the \term{readtable case} of the \term{current readtable},
as outlined in \secref\ReadtableCaseReadEffect.
\itemitem{--}
\param{Y} is appended to the \term{token} being built.
\itemitem{--}
Step 8 is repeated.
\endlist
%% 22.1.1 18
\itemitem{\bull}
If \param{y} is a \term{single escape} \term{character}, then the next \term{character},
\param{z}, is read, or an error \oftype{end-of-file} is signaled if at end of file.
\param{Z} is treated as if it is a \term{constituent}
whose only \term{constituent trait} is \term{alphabetic}\meaning{2}.
\param{Z} is appended to the \term{token} being built,
and step 8 is repeated.
%% 22.1.1 19
\itemitem{\bull}
If \param{y} is a \term{multiple escape} \term{character},
then step 9 is entered.
%% 22.1.1 20
\itemitem{\bull}
If \param{y} is an \term{invalid} \term{character},
an error \oftype{reader-error} is signaled.
%% 22.1.1 21
\itemitem{\bull}
If \param{y} is a \term{terminating} \term{macro character},
then it terminates the \term{token}.
First the \term{character} \param{y} is unread (see \funref{unread-char}),
and then step 10 is entered.
%% 22.1.1 22
\itemitem{\bull}
If \param{y} is a \term{whitespace}\meaning{2} \term{character}, then it terminates
the \term{token}. First the \term{character} \param{y} is unread
if appropriate (see \funref{read-preserving-whitespace}),
and then step 10 is entered.
\endlist
%% 22.1.1 23
%% 22.1.1 24
\item{9.}
At this point a \term{token} is being accumulated, and an odd number
of \term{multiple escape} \term{characters} have been encountered.
If at end of file, an error \oftype{end-of-file} is signaled.
Otherwise, a \term{character}, \param{y}, is read, and
one of the following actions is performed according to its \term{syntax type}:
%% 22.1.1 25
\beginlist
\itemitem{\bull}
If \param{y} is a \term{constituent}, macro, or \term{whitespace}\meaning{2} \term{character},
\param{y} is treated as a \term{constituent}
whose only \term{constituent trait} is \term{alphabetic}\meaning{2}.
\param{Y} is appended to the \term{token} being built, and step 9 is repeated.
%% 22.1.1 26
\itemitem{\bull}
If \param{y} is a \term{single escape} \term{character}, then the next \term{character},
\param{z}, is read, or an error \oftype{end-of-file} is signaled if at end of file.
\param{Z} is treated as a \term{constituent}
whose only \term{constituent trait} is \term{alphabetic}\meaning{2}.
\param{Z} is appended to the \term{token} being built,
and step 9 is repeated.
%% 22.1.1 27
\itemitem{\bull}
If \param{y} is a \term{multiple escape} \term{character},
then step 8 is entered.
%% 22.1.1 28
\itemitem{\bull}
If \param{y} is an \term{invalid} \term{character},
an error \oftype{reader-error} is signaled.
\endlist
%% 22.1.1 29
\item{10.}
An entire \term{token} has been accumulated.
The \term{object} represented by the \term{token} is returned
as the result of the read operation,
or an error \oftype{reader-error} is signaled if the \term{token} is not of valid syntax.
\endlist
%% 22.1.1 30
%% 22.1.1 31
%%Barmar observes that this is said elsewhere, and in any case is
%%implied by the algorithm above:
% \term{Single escape} and \term{multiple escape} \term{characters}
% can be included in a \term{token} when
% preceded by another \term{single escape} \term{character}.