| rev |
line source |
|
jbe@37
|
1 seqlua: Extension for handling sequential data in Lua
|
|
jbe@37
|
2 =====================================================
|
|
jbe@0
|
3
|
|
jbe@49
|
4 This package is an extension for the Lua language which:
|
|
jbe@0
|
5
|
|
jbe@49
|
6 * allows ``ipairs(seq)`` to accept either tables or functions (i.e function
|
|
jbe@38
|
7 iterators) as an argument,
|
|
jbe@49
|
8 * adds a new function ``string.concat(separator, seq)`` that concats either
|
|
jbe@32
|
9 table entries or function return values,
|
|
jbe@49
|
10 * provides 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@49
|
21 In Lua, this extension is loaded by ``require "seqlua"``. In order to use the
|
|
jbe@49
|
22 auxiliary C functions and macros, add ``#include <seqlualib.h>`` to your C file
|
|
jbe@49
|
23 and ensure that the functions implemented in ``seqlualib.c`` are statically or
|
|
jbe@49
|
24 dynamically linked with your C Lua library.
|
|
jbe@49
|
25
|
|
jbe@37
|
26
|
|
jbe@37
|
27
|
|
jbe@37
|
28 Motivation
|
|
jbe@37
|
29 ----------
|
|
jbe@37
|
30
|
|
jbe@37
|
31 Sequential data (such as arrays or streams) is often represented in two
|
|
jbe@37
|
32 different ways:
|
|
jbe@37
|
33
|
|
jbe@37
|
34 * as an ordered set of values (usually implemented as an array in other
|
|
jbe@37
|
35 programming languages, or as a sequence in Lua: a table with numeric keys
|
|
jbe@37
|
36 {1..n} associated with a value each),
|
|
jbe@37
|
37 * as some sort of data stream (sometimes implemented as a class of objects
|
|
jbe@37
|
38 providing certain methods, or as an iterator function in Lua: a function that
|
|
jbe@37
|
39 returns the next value with every call, where nil indicates the end of the
|
|
jbe@37
|
40 stream).
|
|
jbe@37
|
41
|
|
jbe@37
|
42 Quite often, when functions work on sequential data, it shouldn't matter in
|
|
jbe@37
|
43 which form the sequential data is being provided to the function. As an
|
|
jbe@37
|
44 example, consider a function that is writing a sequence of strings to a file.
|
|
jbe@37
|
45 Such function could either be fed with an array of strings (a table with
|
|
jbe@37
|
46 numeric keys in Lua) or with a (possibly infinite) stream of data (an iterator
|
|
jbe@37
|
47 function in Lua).
|
|
jbe@37
|
48
|
|
jbe@37
|
49 A function in Lua that accepts a table, might look like as follows:
|
|
jbe@37
|
50
|
|
jbe@37
|
51 function write_lines(lines)
|
|
jbe@37
|
52 for i, line in ipairs(lines) do
|
|
jbe@37
|
53 io.stdout:write(line)
|
|
jbe@37
|
54 io.stdout:write("\n")
|
|
jbe@37
|
55 end
|
|
jbe@37
|
56 end
|
|
jbe@37
|
57
|
|
jbe@37
|
58 In contrast, a function in Lua that accepts an iterator function would have to
|
|
jbe@37
|
59 be implemented differently:
|
|
jbe@37
|
60
|
|
jbe@37
|
61 function write_lines(get_next_line)
|
|
jbe@37
|
62 for line in get_next_line do
|
|
jbe@37
|
63 io.stdout:write(line)
|
|
jbe@37
|
64 io.stdout:write("\n")
|
|
jbe@37
|
65 end
|
|
jbe@37
|
66 end
|
|
jbe@37
|
67
|
|
jbe@37
|
68 If one wanted to create a function that accepts either a sequence in form of a
|
|
jbe@37
|
69 table or an iterator function, then one might need to write:
|
|
jbe@37
|
70
|
|
jbe@41
|
71 do
|
|
jbe@41
|
72 local function write_line(line)
|
|
jbe@37
|
73 io.stdout:write(line)
|
|
jbe@37
|
74 io.stdout:write("\n")
|
|
jbe@37
|
75 end
|
|
jbe@41
|
76 function write_lines(lines)
|
|
jbe@41
|
77 if type(lines) == "function" then
|
|
jbe@41
|
78 for line in lines do
|
|
jbe@41
|
79 write_line(line)
|
|
jbe@41
|
80 end
|
|
jbe@41
|
81 else
|
|
jbe@41
|
82 for i, line in ipairs(lines) do
|
|
jbe@41
|
83 write_line(line)
|
|
jbe@41
|
84 end
|
|
jbe@41
|
85 end
|
|
jbe@41
|
86 end
|
|
jbe@37
|
87 end
|
|
jbe@37
|
88
|
|
jbe@41
|
89 Obviously, this isn't something we want to do in every function that accepts
|
|
jbe@37
|
90 sequential data. Therefore, we usually decide for one of the two first forms
|
|
jbe@48
|
91 and thus disallow the other possible representation of sequential data to be
|
|
jbe@48
|
92 passed to the function.
|
|
jbe@37
|
93
|
|
jbe@37
|
94 This extension, however, modifies Lua's ``ipairs`` statement in such way that
|
|
jbe@37
|
95 it automatically accepts either a table or an iterator function as argument.
|
|
jbe@42
|
96 Thus, the first of the three ``write_lines`` functions above will accept both
|
|
jbe@42
|
97 (table) sequences and (function) iterators.
|
|
jbe@37
|
98
|
|
jbe@37
|
99 In addition to the modification of ``ipairs``, it also provides C functions and
|
|
jbe@37
|
100 macros to iterate over values in the same manner as a generic loop statement
|
|
jbe@37
|
101 with ``ipairs`` would do.
|
|
jbe@37
|
102
|
|
jbe@37
|
103 Note that this extension doesn't aim to supersede Lua's concept of iterator
|
|
jbe@37
|
104 functions. While metamethods (see section "Respected metamethods" below) may be
|
|
jbe@37
|
105 used to customize iteration behavior on values, this extension isn't thought to
|
|
jbe@37
|
106 replace the common practice to use function closures as iterators. Consider the
|
|
jbe@37
|
107 following example:
|
|
jbe@37
|
108
|
|
jbe@37
|
109 local result = sql_query("SELECT * FROM actor ORDER BY birthdate")
|
|
jbe@37
|
110 write_lines(result:get_column_entries("name"))
|
|
jbe@37
|
111
|
|
jbe@37
|
112 The ``get_column_entries`` method can return a simple function closure that
|
|
jbe@37
|
113 returns the next entry in the "name" column (returning ``nil`` to indicate the
|
|
jbe@37
|
114 end). Such a closure can then be passed to another function that iterates
|
|
jbe@37
|
115 through a sequence of values by invoking ``ipairs`` with the general for-loop
|
|
jbe@37
|
116 (as previously shown).
|
|
jbe@37
|
117
|
|
jbe@37
|
118 Where desired, it is also possible to use metamethods to customize iteration
|
|
jbe@44
|
119 behavior:
|
|
jbe@44
|
120
|
|
jbe@44
|
121 function print_rows(rows)
|
|
jbe@44
|
122 for i, row in ipairs(rows) do
|
|
jbe@44
|
123 print_row(row)
|
|
jbe@44
|
124 end
|
|
jbe@44
|
125 end
|
|
jbe@44
|
126 local result = sql_query("SELECT * FROM actor ORDER BY birthday")
|
|
jbe@46
|
127 assert(type(result) == "userdata")
|
|
jbe@44
|
128
|
|
jbe@44
|
129 -- we may rely on the ``__index`` or ``__ipairs`` metamethod to
|
|
jbe@44
|
130 -- iterate through all result rows here:
|
|
jbe@44
|
131 print_rows(result) -- no need to use ":rows()" or a similar syntax
|
|
jbe@44
|
132
|
|
jbe@45
|
133 -- but we can also still pass an individual set of result rows to the
|
|
jbe@44
|
134 -- print_rows function:
|
|
jbe@44
|
135 print_rows{result[1], result[#result]}
|
|
jbe@44
|
136
|
|
jbe@44
|
137 This extension, however, doesn't respect the ``__len`` metamethod due to the
|
|
jbe@47
|
138 following considerations:
|
|
jbe@37
|
139
|
|
jbe@39
|
140 * An efficient implementation where ``for i, v in ipairs(tbl) do ... end`` does
|
|
jbe@39
|
141 neither create a closure nor repeatedly evaluate ``#tbl`` seems to be
|
|
jbe@39
|
142 impossible.
|
|
jbe@37
|
143 * Respecting ``__len`` could be used to implement sparse arrays, but this would
|
|
jbe@37
|
144 require iterating functions to expect ``nil`` as a potential value. This may
|
|
jbe@37
|
145 lead to problems because ``nil`` is usually also used to indicate the absence
|
|
jbe@37
|
146 of a value.
|
|
jbe@37
|
147
|
|
jbe@40
|
148 Though, if such behavior is desired, it can still be implemented through the
|
|
jbe@37
|
149 ``__ipairs`` metamethod.
|
|
jbe@37
|
150
|
|
jbe@48
|
151 Unless manually done by the user in the ``__ipairs`` metamethod, the ``ipairs``
|
|
jbe@48
|
152 function as well as the corresponding C functions and macros provided by this
|
|
jbe@48
|
153 extension never create any closures or other values that need to be garbage
|
|
jbe@48
|
154 collected.
|
|
jbe@37
|
155
|
|
jbe@0
|
156
|
|
jbe@0
|
157
|
|
jbe@0
|
158 Lua part of the library
|
|
jbe@0
|
159 -----------------------
|
|
jbe@0
|
160
|
|
jbe@30
|
161 The modified ``ipairs(seq)`` and the new ``string.concat(sep, seq)`` functions
|
|
jbe@30
|
162 accept either a table or a function as ``seq``. This is demonstrated in the
|
|
jbe@30
|
163 following examples:
|
|
jbe@0
|
164
|
|
jbe@0
|
165 require "seqlua"
|
|
jbe@0
|
166
|
|
jbe@0
|
167 t = {"a", "b", "c"}
|
|
jbe@0
|
168
|
|
jbe@0
|
169 for i, v in ipairs(t) do
|
|
jbe@0
|
170 print(i, v)
|
|
jbe@0
|
171 end
|
|
jbe@0
|
172 -- prints:
|
|
jbe@0
|
173 -- 1 a
|
|
jbe@0
|
174 -- 2 b
|
|
jbe@0
|
175 -- 3 c
|
|
jbe@0
|
176
|
|
jbe@25
|
177 print(string.concat(",", t))
|
|
jbe@25
|
178 -- prints: a,b,c
|
|
jbe@25
|
179
|
|
jbe@19
|
180 function alphabet()
|
|
jbe@0
|
181 local letter = nil
|
|
jbe@0
|
182 return function()
|
|
jbe@0
|
183 if letter == nil then
|
|
jbe@19
|
184 letter = "a"
|
|
jbe@19
|
185 elseif letter == "z" then
|
|
jbe@0
|
186 return nil
|
|
jbe@0
|
187 else
|
|
jbe@0
|
188 letter = string.char(string.byte(letter) + 1)
|
|
jbe@0
|
189 end
|
|
jbe@0
|
190 return letter
|
|
jbe@0
|
191 end
|
|
jbe@0
|
192 end
|
|
jbe@0
|
193
|
|
jbe@23
|
194 for i, v in ipairs(alphabet()) do
|
|
jbe@0
|
195 print(i, v)
|
|
jbe@0
|
196 end
|
|
jbe@0
|
197 -- prints:
|
|
jbe@0
|
198 -- 1 a
|
|
jbe@0
|
199 -- 2 b
|
|
jbe@0
|
200 -- 3 c
|
|
jbe@0
|
201 -- ...
|
|
jbe@0
|
202 -- 25 y
|
|
jbe@0
|
203 -- 26 z
|
|
jbe@0
|
204
|
|
jbe@25
|
205 print(string.concat(",", alphabet()))
|
|
jbe@25
|
206 -- 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
|
207
|
|
jbe@26
|
208 function filter(f)
|
|
jbe@26
|
209 return function(seq)
|
|
jbe@26
|
210 return coroutine.wrap(function()
|
|
jbe@26
|
211 for i, v in ipairs(seq) do f(v) end
|
|
jbe@26
|
212 end)
|
|
jbe@26
|
213 end
|
|
jbe@0
|
214 end
|
|
jbe@19
|
215
|
|
jbe@29
|
216 alpha_beta_x = filter(function(v)
|
|
jbe@28
|
217 if v == "a" then
|
|
jbe@28
|
218 coroutine.yield("alpha")
|
|
jbe@28
|
219 elseif v == "b" then
|
|
jbe@28
|
220 coroutine.yield("beta")
|
|
jbe@28
|
221 elseif type(v) == "number" then
|
|
jbe@23
|
222 for i = 1, v do
|
|
jbe@28
|
223 coroutine.yield("X")
|
|
jbe@23
|
224 end
|
|
jbe@0
|
225 end
|
|
jbe@26
|
226 end)
|
|
jbe@0
|
227
|
|
jbe@29
|
228 print((","):concat(alpha_beta_x{"a", 3, "b", "c", "d"}))
|
|
jbe@28
|
229 -- prints: alpha,X,X,X,beta
|
|
jbe@25
|
230
|
|
jbe@29
|
231 print((","):concat(alpha_beta_x(alphabet())))
|
|
jbe@28
|
232 -- prints: alpha,beta
|
|
jbe@27
|
233
|
|
jbe@0
|
234
|
|
jbe@37
|
235
|
|
jbe@0
|
236 C part of the library
|
|
jbe@0
|
237 ---------------------
|
|
jbe@0
|
238
|
|
jbe@0
|
239 In ``seqlualib.h``, the following macro is defined:
|
|
jbe@0
|
240
|
|
jbe@0
|
241 #define seqlua_iterloop(L, iter, idx) \
|
|
jbe@0
|
242 for ( \
|
|
jbe@0
|
243 seqlua_iterinit((L), (iter), (idx)); \
|
|
jbe@0
|
244 seqlua_iternext(iter); \
|
|
jbe@25
|
245 )
|
|
jbe@25
|
246
|
|
jbe@25
|
247 and
|
|
jbe@25
|
248
|
|
jbe@25
|
249 #define seqlua_iterloopauto(L, iter, idx) \
|
|
jbe@25
|
250 for ( \
|
|
jbe@25
|
251 seqlua_iterinit((L), (iter), (idx)); \
|
|
jbe@25
|
252 seqlua_iternext(iter); \
|
|
jbe@0
|
253 lua_pop((L), 1) \
|
|
jbe@0
|
254 )
|
|
jbe@0
|
255
|
|
jbe@23
|
256 This macro allows iteration over either tables or iterator functions as the
|
|
jbe@23
|
257 following example function demonstrates:
|
|
jbe@0
|
258
|
|
jbe@0
|
259 int printcsv(lua_State *L) {
|
|
jbe@0
|
260 seqlua_Iterator iter;
|
|
jbe@0
|
261 seqlua_iterloop(L, &iter, 1) {
|
|
jbe@0
|
262 if (seqlua_itercount(&iter) > 1) fputs(",", stdout);
|
|
jbe@0
|
263 fputs(luaL_tolstring(L, -1, NULL), stdout);
|
|
jbe@25
|
264 // two values need to be popped (the value pushed by
|
|
jbe@25
|
265 // seqlua_iternext and the value pushed by luaL_tolstring)
|
|
jbe@25
|
266 lua_pop(L, 2);
|
|
jbe@0
|
267 }
|
|
jbe@0
|
268 fputs("\n", stdout);
|
|
jbe@0
|
269 return 0;
|
|
jbe@0
|
270 }
|
|
jbe@0
|
271
|
|
jbe@11
|
272 printcsv{"a", "b", "c"}
|
|
jbe@11
|
273 -- prints: a,b,c
|
|
jbe@11
|
274
|
|
jbe@11
|
275 printcsv(assert(io.open("testfile")):lines())
|
|
jbe@11
|
276 -- prints: line1,line2,... of "testfile"
|
|
jbe@0
|
277
|
|
jbe@31
|
278 NOTE: During iteration using ``seqlua_iterloop``, ``seqlua_iterloopauto``, or
|
|
jbe@31
|
279 ``seqlua_iterinit``, three extra elements are stored on the stack (additionally
|
|
jbe@31
|
280 to the value). These extra elements are removed automatically when the loop ends
|
|
jbe@31
|
281 (i.e. when ``seqlua_iternext`` returns zero). The value pushed onto the stack
|
|
jbe@31
|
282 for every iteration step has to be removed manually from the stack, unless
|
|
jbe@31
|
283 ``seqlua_iterloopauto`` is used.
|
|
jbe@0
|
284
|
|
jbe@31
|
285
|
|
jbe@37
|
286
|
|
jbe@35
|
287 Respected metamethods
|
|
jbe@35
|
288 ---------------------
|
|
jbe@35
|
289
|
|
jbe@35
|
290 Regarding the behavior of the Lua functions and the C functions and macros
|
|
jbe@35
|
291 provided by this extension, an existing ``__index`` metamethod will be
|
|
jbe@35
|
292 respected automatically. An existing ``__ipairs`` metamethod, however, takes
|
|
jbe@35
|
293 precedence.
|
|
jbe@35
|
294
|
|
jbe@35
|
295 If the ``__ipairs`` field of a value's metatable is set, then it must always
|
|
jbe@35
|
296 refer to a function. When starting iteration over a value with such a
|
|
jbe@35
|
297 metamethod being set, then this function is called with ``self`` (i.e. the
|
|
jbe@35
|
298 value itself) passed as first argument. The return values of the ``__ipairs``
|
|
jbe@35
|
299 metamethod may take one of the following 4 forms:
|
|
jbe@35
|
300
|
|
jbe@35
|
301 * ``return function_or_callable, static_argument, startindex`` causes the three
|
|
jbe@35
|
302 arguments to be returned by ``ipairs`` without further modification. Using
|
|
jbe@35
|
303 the C macros and functions for iteration, the behavior is according to the
|
|
jbe@35
|
304 generic loop statement in Lua:
|
|
jbe@35
|
305 ``for i, v in function_or_callable, static_argument, startindex do ... end``
|
|
jbe@35
|
306 * ``return "raw", table`` will result in iteration over the table ``table``
|
|
jbe@35
|
307 using ``lua_rawgeti``
|
|
jbe@35
|
308 * ``return "index", table_or_userdata`` will result in iteration over the table
|
|
jbe@35
|
309 or userdata while respecting any ``__index`` metamethod of the table or
|
|
jbe@35
|
310 userdata value
|
|
jbe@35
|
311 * ``return "call", function_or_callable`` will use the callable value as
|
|
jbe@35
|
312 (function) iterator where the function is expected to return a single value
|
|
jbe@35
|
313 without any index (the index is inserted automatically when using the
|
|
jbe@35
|
314 ``ipairs`` function for iteration)
|
|
jbe@35
|
315
|
|
jbe@35
|
316 These possiblities are demonstrated by the following example code:
|
|
jbe@35
|
317
|
|
jbe@35
|
318 require "seqlua"
|
|
jbe@35
|
319
|
|
jbe@35
|
320 do
|
|
jbe@35
|
321 local function ipairsaux(t, i)
|
|
jbe@35
|
322 i = i + 1
|
|
jbe@35
|
323 if i <= 3 then
|
|
jbe@35
|
324 return i, t[i]
|
|
jbe@35
|
325 end
|
|
jbe@35
|
326 end
|
|
jbe@35
|
327 custom = setmetatable(
|
|
jbe@35
|
328 {"one", "two", "three", "four", "five"},
|
|
jbe@35
|
329 {
|
|
jbe@35
|
330 __ipairs = function(self)
|
|
jbe@35
|
331 return ipairsaux, self, 0
|
|
jbe@35
|
332 end
|
|
jbe@35
|
333 }
|
|
jbe@35
|
334 )
|
|
jbe@35
|
335 end
|
|
jbe@35
|
336 print(string.concat(",", custom))
|
|
jbe@36
|
337 -- prints: one,two,three
|
|
jbe@35
|
338 -- (note that "four" and "five" are not printed)
|
|
jbe@35
|
339
|
|
jbe@35
|
340 tbl = {"alpha", "beta"}
|
|
jbe@35
|
341
|
|
jbe@35
|
342 proxy1 = setmetatable({}, {__index = tbl})
|
|
jbe@35
|
343 for i, v in ipairs(proxy1) do print(i, v) end
|
|
jbe@35
|
344 -- prints:
|
|
jbe@35
|
345 -- 1 alpha
|
|
jbe@35
|
346 -- 2 beta
|
|
jbe@35
|
347
|
|
jbe@35
|
348 proxy2 = setmetatable({}, {
|
|
jbe@35
|
349 __ipairs = function(self)
|
|
jbe@35
|
350 return "index", proxy1
|
|
jbe@35
|
351 end
|
|
jbe@35
|
352 })
|
|
jbe@35
|
353 for i, v in ipairs(proxy2) do print(i, v) end
|
|
jbe@35
|
354 -- prints:
|
|
jbe@35
|
355 -- 1 alpha
|
|
jbe@35
|
356 -- 2 beta
|
|
jbe@35
|
357 print(proxy2[1])
|
|
jbe@35
|
358 -- prints: nil
|
|
jbe@35
|
359
|
|
jbe@35
|
360 cursor = setmetatable({
|
|
jbe@35
|
361 "alice", "bob", "charlie", pos=1
|
|
jbe@35
|
362 }, {
|
|
jbe@35
|
363 __call = function(self)
|
|
jbe@35
|
364 local value = self[self.pos]
|
|
jbe@35
|
365 if value == nil then
|
|
jbe@35
|
366 self.pos = 1
|
|
jbe@35
|
367 else
|
|
jbe@35
|
368 self.pos = self.pos + 1
|
|
jbe@35
|
369 end
|
|
jbe@35
|
370 return value
|
|
jbe@35
|
371 end,
|
|
jbe@35
|
372 __ipairs = function(self)
|
|
jbe@35
|
373 return "call", self
|
|
jbe@35
|
374 end
|
|
jbe@35
|
375 })
|
|
jbe@35
|
376 for i, v in ipairs(cursor) do print(i, v) end
|
|
jbe@35
|
377 -- prints:
|
|
jbe@35
|
378 -- 1 alice
|
|
jbe@35
|
379 -- 2 bob
|
|
jbe@35
|
380 -- 3 charlie
|
|
jbe@35
|
381 print(cursor())
|
|
jbe@35
|
382 -- prints: alice
|
|
jbe@35
|
383 for i, v in ipairs(cursor) do print(i, v) end
|
|
jbe@35
|
384 -- prints:
|
|
jbe@35
|
385 -- 1 bob
|
|
jbe@35
|
386 -- 2 charlie
|
|
jbe@35
|
387 -- (note that "alice" has been returned earlier)
|
|
jbe@35
|
388
|
|
jbe@35
|
389 coefficients = setmetatable({1.25, 3.14, 17.5}, {
|
|
jbe@35
|
390 __index = function(self) return 1 end,
|
|
jbe@35
|
391 __ipairs = function(self) return "raw", self end
|
|
jbe@35
|
392 })
|
|
jbe@35
|
393 for i, v in ipairs(coefficients) do print(i, v) end
|
|
jbe@35
|
394 -- prints:
|
|
jbe@35
|
395 -- 1 1.25
|
|
jbe@35
|
396 -- 2 3.14
|
|
jbe@35
|
397 -- 3 17.5
|
|
jbe@35
|
398 -- (note that iteration terminates even if coefficients[4] == 1)
|
|
jbe@35
|
399 print(coefficients[4])
|
|
jbe@35
|
400 -- prints: 1
|
|
jbe@35
|
401
|
|
jbe@35
|
402
|