| rev |
line source |
|
jbe@37
|
1 seqlua: Extension for handling sequential data in Lua
|
|
jbe@37
|
2 =====================================================
|
|
jbe@0
|
3
|
|
jbe@0
|
4 This is an experimental package to extend Lua in the following manner:
|
|
jbe@0
|
5
|
|
jbe@38
|
6 * allow ``ipairs(seq)`` to accept either tables or functions (i.e function
|
|
jbe@38
|
7 iterators) as an argument,
|
|
jbe@38
|
8 * add a new function ``string.concat(separator, seq)`` that concats either
|
|
jbe@32
|
9 table entries or function return values,
|
|
jbe@23
|
10 * provide auxiliary C functions and macros to simplify iterating over both
|
|
jbe@38
|
11 tables and iterator functions with a generic statement.
|
|
jbe@0
|
12
|
|
jbe@33
|
13 Existing ``__ipairs`` or ``__index`` (but not ``__len``) metamethods are
|
|
jbe@33
|
14 respected by both the Lua functions and the C functions and macros. The
|
|
jbe@32
|
15 ``__ipairs`` metamethod takes precedence over ``__index``, while the
|
|
jbe@32
|
16 ``__len`` metamethod is never used.
|
|
jbe@32
|
17
|
|
jbe@37
|
18 Metamethod handling in detail is explained in the last section
|
|
jbe@37
|
19 ("Respected metamethods") at the bottom of this README.
|
|
jbe@37
|
20
|
|
jbe@37
|
21
|
|
jbe@37
|
22
|
|
jbe@37
|
23 Motivation
|
|
jbe@37
|
24 ----------
|
|
jbe@37
|
25
|
|
jbe@37
|
26 Sequential data (such as arrays or streams) is often represented in two
|
|
jbe@37
|
27 different ways:
|
|
jbe@37
|
28
|
|
jbe@37
|
29 * as an ordered set of values (usually implemented as an array in other
|
|
jbe@37
|
30 programming languages, or as a sequence in Lua: a table with numeric keys
|
|
jbe@37
|
31 {1..n} associated with a value each),
|
|
jbe@37
|
32 * as some sort of data stream (sometimes implemented as a class of objects
|
|
jbe@37
|
33 providing certain methods, or as an iterator function in Lua: a function that
|
|
jbe@37
|
34 returns the next value with every call, where nil indicates the end of the
|
|
jbe@37
|
35 stream).
|
|
jbe@37
|
36
|
|
jbe@37
|
37 Quite often, when functions work on sequential data, it shouldn't matter in
|
|
jbe@37
|
38 which form the sequential data is being provided to the function. As an
|
|
jbe@37
|
39 example, consider a function that is writing a sequence of strings to a file.
|
|
jbe@37
|
40 Such function could either be fed with an array of strings (a table with
|
|
jbe@37
|
41 numeric keys in Lua) or with a (possibly infinite) stream of data (an iterator
|
|
jbe@37
|
42 function in Lua).
|
|
jbe@37
|
43
|
|
jbe@37
|
44 A function in Lua that accepts a table, might look like as follows:
|
|
jbe@37
|
45
|
|
jbe@37
|
46 function write_lines(lines)
|
|
jbe@37
|
47 for i, line in ipairs(lines) do
|
|
jbe@37
|
48 io.stdout:write(line)
|
|
jbe@37
|
49 io.stdout:write("\n")
|
|
jbe@37
|
50 end
|
|
jbe@37
|
51 end
|
|
jbe@37
|
52
|
|
jbe@37
|
53 In contrast, a function in Lua that accepts an iterator function would have to
|
|
jbe@37
|
54 be implemented differently:
|
|
jbe@37
|
55
|
|
jbe@37
|
56 function write_lines(get_next_line)
|
|
jbe@37
|
57 for line in get_next_line do
|
|
jbe@37
|
58 io.stdout:write(line)
|
|
jbe@37
|
59 io.stdout:write("\n")
|
|
jbe@37
|
60 end
|
|
jbe@37
|
61 end
|
|
jbe@37
|
62
|
|
jbe@37
|
63 If one wanted to create a function that accepts either a sequence in form of a
|
|
jbe@37
|
64 table or an iterator function, then one might need to write:
|
|
jbe@37
|
65
|
|
jbe@41
|
66 do
|
|
jbe@41
|
67 local function write_line(line)
|
|
jbe@37
|
68 io.stdout:write(line)
|
|
jbe@37
|
69 io.stdout:write("\n")
|
|
jbe@37
|
70 end
|
|
jbe@41
|
71 function write_lines(lines)
|
|
jbe@41
|
72 if type(lines) == "function" then
|
|
jbe@41
|
73 for line in lines do
|
|
jbe@41
|
74 write_line(line)
|
|
jbe@41
|
75 end
|
|
jbe@41
|
76 else
|
|
jbe@41
|
77 for i, line in ipairs(lines) do
|
|
jbe@41
|
78 write_line(line)
|
|
jbe@41
|
79 end
|
|
jbe@41
|
80 end
|
|
jbe@41
|
81 end
|
|
jbe@37
|
82 end
|
|
jbe@37
|
83
|
|
jbe@41
|
84 Obviously, this isn't something we want to do in every function that accepts
|
|
jbe@37
|
85 sequential data. Therefore, we usually decide for one of the two first forms
|
|
jbe@37
|
86 and therefore disallow the other possible representation of sequential data to
|
|
jbe@37
|
87 be passed to the function.
|
|
jbe@37
|
88
|
|
jbe@37
|
89 This extension, however, modifies Lua's ``ipairs`` statement in such way that
|
|
jbe@37
|
90 it automatically accepts either a table or an iterator function as argument.
|
|
jbe@42
|
91 Thus, the first of the three ``write_lines`` functions above will accept both
|
|
jbe@42
|
92 (table) sequences and (function) iterators.
|
|
jbe@37
|
93
|
|
jbe@37
|
94 In addition to the modification of ``ipairs``, it also provides C functions and
|
|
jbe@37
|
95 macros to iterate over values in the same manner as a generic loop statement
|
|
jbe@37
|
96 with ``ipairs`` would do.
|
|
jbe@37
|
97
|
|
jbe@37
|
98 Note that this extension doesn't aim to supersede Lua's concept of iterator
|
|
jbe@37
|
99 functions. While metamethods (see section "Respected metamethods" below) may be
|
|
jbe@37
|
100 used to customize iteration behavior on values, this extension isn't thought to
|
|
jbe@37
|
101 replace the common practice to use function closures as iterators. Consider the
|
|
jbe@37
|
102 following example:
|
|
jbe@37
|
103
|
|
jbe@37
|
104 local result = sql_query("SELECT * FROM actor ORDER BY birthdate")
|
|
jbe@37
|
105 write_lines(result:get_column_entries("name"))
|
|
jbe@37
|
106
|
|
jbe@37
|
107 The ``get_column_entries`` method can return a simple function closure that
|
|
jbe@37
|
108 returns the next entry in the "name" column (returning ``nil`` to indicate the
|
|
jbe@37
|
109 end). Such a closure can then be passed to another function that iterates
|
|
jbe@37
|
110 through a sequence of values by invoking ``ipairs`` with the general for-loop
|
|
jbe@37
|
111 (as previously shown).
|
|
jbe@37
|
112
|
|
jbe@37
|
113 Where desired, it is also possible to use metamethods to customize iteration
|
|
jbe@44
|
114 behavior:
|
|
jbe@44
|
115
|
|
jbe@44
|
116 function print_rows(rows)
|
|
jbe@44
|
117 for i, row in ipairs(rows) do
|
|
jbe@44
|
118 print_row(row)
|
|
jbe@44
|
119 end
|
|
jbe@44
|
120 end
|
|
jbe@44
|
121 local result = sql_query("SELECT * FROM actor ORDER BY birthday")
|
|
jbe@44
|
122
|
|
jbe@44
|
123 -- we may rely on the ``__index`` or ``__ipairs`` metamethod to
|
|
jbe@44
|
124 -- iterate through all result rows here:
|
|
jbe@44
|
125 print_rows(result) -- no need to use ":rows()" or a similar syntax
|
|
jbe@44
|
126
|
|
jbe@44
|
127 -- but we can also still pass individual result rows to the
|
|
jbe@44
|
128 -- print_rows function:
|
|
jbe@44
|
129 print_rows{result[1], result[#result]}
|
|
jbe@44
|
130
|
|
jbe@44
|
131 This extension, however, doesn't respect the ``__len`` metamethod due to the
|
|
jbe@44
|
132 following reasons:
|
|
jbe@37
|
133
|
|
jbe@39
|
134 * An efficient implementation where ``for i, v in ipairs(tbl) do ... end`` does
|
|
jbe@39
|
135 neither create a closure nor repeatedly evaluate ``#tbl`` seems to be
|
|
jbe@39
|
136 impossible.
|
|
jbe@37
|
137 * Respecting ``__len`` could be used to implement sparse arrays, but this would
|
|
jbe@37
|
138 require iterating functions to expect ``nil`` as a potential value. This may
|
|
jbe@37
|
139 lead to problems because ``nil`` is usually also used to indicate the absence
|
|
jbe@37
|
140 of a value.
|
|
jbe@37
|
141
|
|
jbe@40
|
142 Though, if such behavior is desired, it can still be implemented through the
|
|
jbe@37
|
143 ``__ipairs`` metamethod.
|
|
jbe@37
|
144
|
|
jbe@37
|
145 Unless manually done by the user in the ``__ipairs`` metamethod, this extension
|
|
jbe@37
|
146 never creates any closures or other values that need to be garbage collected.
|
|
jbe@37
|
147
|
|
jbe@0
|
148
|
|
jbe@0
|
149
|
|
jbe@0
|
150 Lua part of the library
|
|
jbe@0
|
151 -----------------------
|
|
jbe@0
|
152
|
|
jbe@30
|
153 The modified ``ipairs(seq)`` and the new ``string.concat(sep, seq)`` functions
|
|
jbe@30
|
154 accept either a table or a function as ``seq``. This is demonstrated in the
|
|
jbe@30
|
155 following examples:
|
|
jbe@0
|
156
|
|
jbe@0
|
157 require "seqlua"
|
|
jbe@0
|
158
|
|
jbe@0
|
159 t = {"a", "b", "c"}
|
|
jbe@0
|
160
|
|
jbe@0
|
161 for i, v in ipairs(t) do
|
|
jbe@0
|
162 print(i, v)
|
|
jbe@0
|
163 end
|
|
jbe@0
|
164 -- prints:
|
|
jbe@0
|
165 -- 1 a
|
|
jbe@0
|
166 -- 2 b
|
|
jbe@0
|
167 -- 3 c
|
|
jbe@0
|
168
|
|
jbe@25
|
169 print(string.concat(",", t))
|
|
jbe@25
|
170 -- prints: a,b,c
|
|
jbe@25
|
171
|
|
jbe@19
|
172 function alphabet()
|
|
jbe@0
|
173 local letter = nil
|
|
jbe@0
|
174 return function()
|
|
jbe@0
|
175 if letter == nil then
|
|
jbe@19
|
176 letter = "a"
|
|
jbe@19
|
177 elseif letter == "z" then
|
|
jbe@0
|
178 return nil
|
|
jbe@0
|
179 else
|
|
jbe@0
|
180 letter = string.char(string.byte(letter) + 1)
|
|
jbe@0
|
181 end
|
|
jbe@0
|
182 return letter
|
|
jbe@0
|
183 end
|
|
jbe@0
|
184 end
|
|
jbe@0
|
185
|
|
jbe@23
|
186 for i, v in ipairs(alphabet()) do
|
|
jbe@0
|
187 print(i, v)
|
|
jbe@0
|
188 end
|
|
jbe@0
|
189 -- prints:
|
|
jbe@0
|
190 -- 1 a
|
|
jbe@0
|
191 -- 2 b
|
|
jbe@0
|
192 -- 3 c
|
|
jbe@0
|
193 -- ...
|
|
jbe@0
|
194 -- 25 y
|
|
jbe@0
|
195 -- 26 z
|
|
jbe@0
|
196
|
|
jbe@25
|
197 print(string.concat(",", alphabet()))
|
|
jbe@25
|
198 -- prints: a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z
|
|
jbe@25
|
199
|
|
jbe@26
|
200 function filter(f)
|
|
jbe@26
|
201 return function(seq)
|
|
jbe@26
|
202 return coroutine.wrap(function()
|
|
jbe@26
|
203 for i, v in ipairs(seq) do f(v) end
|
|
jbe@26
|
204 end)
|
|
jbe@26
|
205 end
|
|
jbe@0
|
206 end
|
|
jbe@19
|
207
|
|
jbe@29
|
208 alpha_beta_x = filter(function(v)
|
|
jbe@28
|
209 if v == "a" then
|
|
jbe@28
|
210 coroutine.yield("alpha")
|
|
jbe@28
|
211 elseif v == "b" then
|
|
jbe@28
|
212 coroutine.yield("beta")
|
|
jbe@28
|
213 elseif type(v) == "number" then
|
|
jbe@23
|
214 for i = 1, v do
|
|
jbe@28
|
215 coroutine.yield("X")
|
|
jbe@23
|
216 end
|
|
jbe@0
|
217 end
|
|
jbe@26
|
218 end)
|
|
jbe@0
|
219
|
|
jbe@29
|
220 print((","):concat(alpha_beta_x{"a", 3, "b", "c", "d"}))
|
|
jbe@28
|
221 -- prints: alpha,X,X,X,beta
|
|
jbe@25
|
222
|
|
jbe@29
|
223 print((","):concat(alpha_beta_x(alphabet())))
|
|
jbe@28
|
224 -- prints: alpha,beta
|
|
jbe@27
|
225
|
|
jbe@0
|
226
|
|
jbe@37
|
227
|
|
jbe@0
|
228 C part of the library
|
|
jbe@0
|
229 ---------------------
|
|
jbe@0
|
230
|
|
jbe@0
|
231 In ``seqlualib.h``, the following macro is defined:
|
|
jbe@0
|
232
|
|
jbe@0
|
233 #define seqlua_iterloop(L, iter, idx) \
|
|
jbe@0
|
234 for ( \
|
|
jbe@0
|
235 seqlua_iterinit((L), (iter), (idx)); \
|
|
jbe@0
|
236 seqlua_iternext(iter); \
|
|
jbe@25
|
237 )
|
|
jbe@25
|
238
|
|
jbe@25
|
239 and
|
|
jbe@25
|
240
|
|
jbe@25
|
241 #define seqlua_iterloopauto(L, iter, idx) \
|
|
jbe@25
|
242 for ( \
|
|
jbe@25
|
243 seqlua_iterinit((L), (iter), (idx)); \
|
|
jbe@25
|
244 seqlua_iternext(iter); \
|
|
jbe@0
|
245 lua_pop((L), 1) \
|
|
jbe@0
|
246 )
|
|
jbe@0
|
247
|
|
jbe@23
|
248 This macro allows iteration over either tables or iterator functions as the
|
|
jbe@23
|
249 following example function demonstrates:
|
|
jbe@0
|
250
|
|
jbe@0
|
251 int printcsv(lua_State *L) {
|
|
jbe@0
|
252 seqlua_Iterator iter;
|
|
jbe@0
|
253 seqlua_iterloop(L, &iter, 1) {
|
|
jbe@0
|
254 if (seqlua_itercount(&iter) > 1) fputs(",", stdout);
|
|
jbe@0
|
255 fputs(luaL_tolstring(L, -1, NULL), stdout);
|
|
jbe@25
|
256 // two values need to be popped (the value pushed by
|
|
jbe@25
|
257 // seqlua_iternext and the value pushed by luaL_tolstring)
|
|
jbe@25
|
258 lua_pop(L, 2);
|
|
jbe@0
|
259 }
|
|
jbe@0
|
260 fputs("\n", stdout);
|
|
jbe@0
|
261 return 0;
|
|
jbe@0
|
262 }
|
|
jbe@0
|
263
|
|
jbe@11
|
264 printcsv{"a", "b", "c"}
|
|
jbe@11
|
265 -- prints: a,b,c
|
|
jbe@11
|
266
|
|
jbe@11
|
267 printcsv(assert(io.open("testfile")):lines())
|
|
jbe@11
|
268 -- prints: line1,line2,... of "testfile"
|
|
jbe@0
|
269
|
|
jbe@31
|
270 NOTE: During iteration using ``seqlua_iterloop``, ``seqlua_iterloopauto``, or
|
|
jbe@31
|
271 ``seqlua_iterinit``, three extra elements are stored on the stack (additionally
|
|
jbe@31
|
272 to the value). These extra elements are removed automatically when the loop ends
|
|
jbe@31
|
273 (i.e. when ``seqlua_iternext`` returns zero). The value pushed onto the stack
|
|
jbe@31
|
274 for every iteration step has to be removed manually from the stack, unless
|
|
jbe@31
|
275 ``seqlua_iterloopauto`` is used.
|
|
jbe@0
|
276
|
|
jbe@31
|
277
|
|
jbe@37
|
278
|
|
jbe@35
|
279 Respected metamethods
|
|
jbe@35
|
280 ---------------------
|
|
jbe@35
|
281
|
|
jbe@35
|
282 Regarding the behavior of the Lua functions and the C functions and macros
|
|
jbe@35
|
283 provided by this extension, an existing ``__index`` metamethod will be
|
|
jbe@35
|
284 respected automatically. An existing ``__ipairs`` metamethod, however, takes
|
|
jbe@35
|
285 precedence.
|
|
jbe@35
|
286
|
|
jbe@35
|
287 If the ``__ipairs`` field of a value's metatable is set, then it must always
|
|
jbe@35
|
288 refer to a function. When starting iteration over a value with such a
|
|
jbe@35
|
289 metamethod being set, then this function is called with ``self`` (i.e. the
|
|
jbe@35
|
290 value itself) passed as first argument. The return values of the ``__ipairs``
|
|
jbe@35
|
291 metamethod may take one of the following 4 forms:
|
|
jbe@35
|
292
|
|
jbe@35
|
293 * ``return function_or_callable, static_argument, startindex`` causes the three
|
|
jbe@35
|
294 arguments to be returned by ``ipairs`` without further modification. Using
|
|
jbe@35
|
295 the C macros and functions for iteration, the behavior is according to the
|
|
jbe@35
|
296 generic loop statement in Lua:
|
|
jbe@35
|
297 ``for i, v in function_or_callable, static_argument, startindex do ... end``
|
|
jbe@35
|
298 * ``return "raw", table`` will result in iteration over the table ``table``
|
|
jbe@35
|
299 using ``lua_rawgeti``
|
|
jbe@35
|
300 * ``return "index", table_or_userdata`` will result in iteration over the table
|
|
jbe@35
|
301 or userdata while respecting any ``__index`` metamethod of the table or
|
|
jbe@35
|
302 userdata value
|
|
jbe@35
|
303 * ``return "call", function_or_callable`` will use the callable value as
|
|
jbe@35
|
304 (function) iterator where the function is expected to return a single value
|
|
jbe@35
|
305 without any index (the index is inserted automatically when using the
|
|
jbe@35
|
306 ``ipairs`` function for iteration)
|
|
jbe@35
|
307
|
|
jbe@35
|
308 These possiblities are demonstrated by the following example code:
|
|
jbe@35
|
309
|
|
jbe@35
|
310 require "seqlua"
|
|
jbe@35
|
311
|
|
jbe@35
|
312 do
|
|
jbe@35
|
313 local function ipairsaux(t, i)
|
|
jbe@35
|
314 i = i + 1
|
|
jbe@35
|
315 if i <= 3 then
|
|
jbe@35
|
316 return i, t[i]
|
|
jbe@35
|
317 end
|
|
jbe@35
|
318 end
|
|
jbe@35
|
319 custom = setmetatable(
|
|
jbe@35
|
320 {"one", "two", "three", "four", "five"},
|
|
jbe@35
|
321 {
|
|
jbe@35
|
322 __ipairs = function(self)
|
|
jbe@35
|
323 return ipairsaux, self, 0
|
|
jbe@35
|
324 end
|
|
jbe@35
|
325 }
|
|
jbe@35
|
326 )
|
|
jbe@35
|
327 end
|
|
jbe@35
|
328 print(string.concat(",", custom))
|
|
jbe@36
|
329 -- prints: one,two,three
|
|
jbe@35
|
330 -- (note that "four" and "five" are not printed)
|
|
jbe@35
|
331
|
|
jbe@35
|
332 tbl = {"alpha", "beta"}
|
|
jbe@35
|
333
|
|
jbe@35
|
334 proxy1 = setmetatable({}, {__index = tbl})
|
|
jbe@35
|
335 for i, v in ipairs(proxy1) do print(i, v) end
|
|
jbe@35
|
336 -- prints:
|
|
jbe@35
|
337 -- 1 alpha
|
|
jbe@35
|
338 -- 2 beta
|
|
jbe@35
|
339
|
|
jbe@35
|
340 proxy2 = setmetatable({}, {
|
|
jbe@35
|
341 __ipairs = function(self)
|
|
jbe@35
|
342 return "index", proxy1
|
|
jbe@35
|
343 end
|
|
jbe@35
|
344 })
|
|
jbe@35
|
345 for i, v in ipairs(proxy2) do print(i, v) end
|
|
jbe@35
|
346 -- prints:
|
|
jbe@35
|
347 -- 1 alpha
|
|
jbe@35
|
348 -- 2 beta
|
|
jbe@35
|
349 print(proxy2[1])
|
|
jbe@35
|
350 -- prints: nil
|
|
jbe@35
|
351
|
|
jbe@35
|
352 cursor = setmetatable({
|
|
jbe@35
|
353 "alice", "bob", "charlie", pos=1
|
|
jbe@35
|
354 }, {
|
|
jbe@35
|
355 __call = function(self)
|
|
jbe@35
|
356 local value = self[self.pos]
|
|
jbe@35
|
357 if value == nil then
|
|
jbe@35
|
358 self.pos = 1
|
|
jbe@35
|
359 else
|
|
jbe@35
|
360 self.pos = self.pos + 1
|
|
jbe@35
|
361 end
|
|
jbe@35
|
362 return value
|
|
jbe@35
|
363 end,
|
|
jbe@35
|
364 __ipairs = function(self)
|
|
jbe@35
|
365 return "call", self
|
|
jbe@35
|
366 end
|
|
jbe@35
|
367 })
|
|
jbe@35
|
368 for i, v in ipairs(cursor) do print(i, v) end
|
|
jbe@35
|
369 -- prints:
|
|
jbe@35
|
370 -- 1 alice
|
|
jbe@35
|
371 -- 2 bob
|
|
jbe@35
|
372 -- 3 charlie
|
|
jbe@35
|
373 print(cursor())
|
|
jbe@35
|
374 -- prints: alice
|
|
jbe@35
|
375 for i, v in ipairs(cursor) do print(i, v) end
|
|
jbe@35
|
376 -- prints:
|
|
jbe@35
|
377 -- 1 bob
|
|
jbe@35
|
378 -- 2 charlie
|
|
jbe@35
|
379 -- (note that "alice" has been returned earlier)
|
|
jbe@35
|
380
|
|
jbe@35
|
381 coefficients = setmetatable({1.25, 3.14, 17.5}, {
|
|
jbe@35
|
382 __index = function(self) return 1 end,
|
|
jbe@35
|
383 __ipairs = function(self) return "raw", self end
|
|
jbe@35
|
384 })
|
|
jbe@35
|
385 for i, v in ipairs(coefficients) do print(i, v) end
|
|
jbe@35
|
386 -- prints:
|
|
jbe@35
|
387 -- 1 1.25
|
|
jbe@35
|
388 -- 2 3.14
|
|
jbe@35
|
389 -- 3 17.5
|
|
jbe@35
|
390 -- (note that iteration terminates even if coefficients[4] == 1)
|
|
jbe@35
|
391 print(coefficients[4])
|
|
jbe@35
|
392 -- prints: 1
|
|
jbe@35
|
393
|
|
jbe@35
|
394
|