diff --git a/README.md b/README.md
index 3505e3a..e6ff772 100644
--- a/README.md
+++ b/README.md
@@ -134,7 +134,21 @@ Other interesting combinations include:
 
 Special thanks to [pfultz2](https://github.com/pfultz2/Cloak/wiki/Is-the-C-preprocessor-Turing-complete%3F) for inventing the EVAL idea.
 
+## Usage
+
+The [map.h](map.h) file defines several "map"-style macros for different use cases. The [example.c](example.c) file shows how to use them. A quick overview is:
+
+C-Code | Evaluates to
+-------| ------------
+MAP(f,a,b,c) | f(a) f(b) f(c)
+MAP_LIST(f,a,b,c) | f(a), f(b), f(c)
+MAP_UD(f,x,a,b,c) | f(a,x) f(b,x) f(c,x)
+MAP_LIST_UD(f,x,a,b,c) | f(a,x), f(b,x), f(c,x)
+MAP_UD_I(f,x,a,b,c) | f(a,x,0) f(b,x,1) f(c,x,2)
+MAP_LIST_UD_I(f,x,a,b,c) | f(a,x,0), f(b,x,1), f(c,x,2)
+
 ## See Also
 
 C++ users may be intersted in the ['visit_struct' library](https://github.com/cbeck88/visit_struct),
-which uses a version of this macro to implement structure visitors in C++11.
+which uses a version of this macro to implement structure visitors in C++11. The ['µSer' library](https://github.com/Erlkoenig90/uSer)
+uses another variant to define serializable structs in C++17.
diff --git a/example.c b/example.c
index 9969293..095e1a6 100644
--- a/example.c
+++ b/example.c
@@ -18,3 +18,19 @@ MAP(CALL, ('a'), ('b'), ('c'))
 /* Test `MAP_LIST` with parentheses in the arguments: */
 #define CALL_LIST(x) putchar x
 MAP_LIST(CALL_LIST, ('a'), ('b'), ('c'));
+
+/* Pass user-defined data to each invocation */
+#define PRINT_STREAM(x, stream) fprintf (stream, "%d\n", x);
+MAP_UD(PRINT_STREAM, stderr, 1, 2, 3, 4, 5)
+
+/* Pass user-defined data to each list invocation */
+#define INVOKE(x, fun) fun(x)
+int arr [] = { MAP_LIST_UD(INVOKE, map, 1, 2, 3, 4, 5) };
+
+/* Pass user-defined data and an index to each invocation */
+#define PRINT_STREAM_I(x, stream, index) fprintf (stream, "%d: %d\n", index, x);
+MAP_UD_I(PRINT_STREAM_I, stderr, 1, 2, 3, 4, 5)
+
+/* Pass user-defined data and an index to each list invocation */
+#define INVOKE_I(x, fun, index) fun(index, x)
+int arr2 [] = { MAP_LIST_UD_I(INVOKE_I, map2, 1, 2, 3, 4, 5) };
diff --git a/map.h b/map.h
index acd5018..46770e8 100644
--- a/map.h
+++ b/map.h
@@ -46,16 +46,29 @@
 #define MAP_NEXT0(test, next, ...) next MAP_OUT
 #define MAP_NEXT1(test, next) MAP_NEXT0(test, next, 0)
 #define MAP_NEXT(test, next)  MAP_NEXT1(MAP_GET_END test, next)
+#define MAP_INC(X) MAP_INC_ ## X
 
 #define MAP0(f, x, peek, ...) f(x) MAP_NEXT(peek, MAP1)(f, peek, __VA_ARGS__)
 #define MAP1(f, x, peek, ...) f(x) MAP_NEXT(peek, MAP0)(f, peek, __VA_ARGS__)
 
+#define MAP0_UD(f, userdata, x, peek, ...) f(x,userdata) MAP_NEXT(peek, MAP1_UD)(f, userdata, peek, __VA_ARGS__)
+#define MAP1_UD(f, userdata, x, peek, ...) f(x,userdata) MAP_NEXT(peek, MAP0_UD)(f, userdata, peek, __VA_ARGS__)
+
+#define MAP0_UD_I(f, userdata, index, x, peek, ...) f(x,userdata,index) MAP_NEXT(peek, MAP1_UD_I)(f, userdata, MAP_INC(index), peek, __VA_ARGS__)
+#define MAP1_UD_I(f, userdata, index, x, peek, ...) f(x,userdata,index) MAP_NEXT(peek, MAP0_UD_I)(f, userdata, MAP_INC(index), peek, __VA_ARGS__)
+
 #define MAP_LIST_NEXT1(test, next) MAP_NEXT0(test, MAP_COMMA next, 0)
 #define MAP_LIST_NEXT(test, next)  MAP_LIST_NEXT1(MAP_GET_END test, next)
 
 #define MAP_LIST0(f, x, peek, ...) f(x) MAP_LIST_NEXT(peek, MAP_LIST1)(f, peek, __VA_ARGS__)
 #define MAP_LIST1(f, x, peek, ...) f(x) MAP_LIST_NEXT(peek, MAP_LIST0)(f, peek, __VA_ARGS__)
 
+#define MAP_LIST0_UD(f, userdata, x, peek, ...) f(x, userdata) MAP_LIST_NEXT(peek, MAP_LIST1_UD)(f, userdata, peek, __VA_ARGS__)
+#define MAP_LIST1_UD(f, userdata, x, peek, ...) f(x, userdata) MAP_LIST_NEXT(peek, MAP_LIST0_UD)(f, userdata, peek, __VA_ARGS__)
+
+#define MAP_LIST0_UD_I(f, userdata, index, x, peek, ...) f(x, userdata, index) MAP_LIST_NEXT(peek, MAP_LIST1_UD_I)(f, userdata, MAP_INC(index), peek, __VA_ARGS__)
+#define MAP_LIST1_UD_I(f, userdata, index, x, peek, ...) f(x, userdata, index) MAP_LIST_NEXT(peek, MAP_LIST0_UD_I)(f, userdata, MAP_INC(index), peek, __VA_ARGS__)
+
 /**
  * Applies the function macro `f` to each of the remaining parameters.
  */
@@ -67,4 +80,408 @@
  */
 #define MAP_LIST(f, ...) EVAL(MAP_LIST1(f, __VA_ARGS__, ()()(), ()()(), ()()(), 0))
 
+/**
+ * Applies the function macro `f` to each of the remaining parameters and passes userdata as the second parameter to each invocation,
+ * e.g. MAP_UD(f, x, a, b, c) evaluates to f(a, x) f(b, x) f(c, x)
+ */
+#define MAP_UD(f, userdata, ...) EVAL(MAP1_UD(f, userdata, __VA_ARGS__, ()()(), ()()(), ()()(), 0))
+
+/**
+ * Applies the function macro `f` to each of the remaining parameters, inserts commas between the results,
+ * and passes userdata as the second parameter to each invocation,
+ * e.g. MAP_LIST_UD(f, x, a, b, c) evaluates to f(a, x), f(b, x), f(c, x)
+ */
+#define MAP_LIST_UD(f, userdata, ...) EVAL(MAP_LIST1_UD(f, userdata, __VA_ARGS__, ()()(), ()()(), ()()(), 0))
+
+/**
+ * Applies the function macro `f` to each of the remaining parameters, passes userdata as the second parameter to each invocation,
+ * and the index of the invocation as the third parameter,
+ * e.g. MAP_UD_I(f, x, a, b, c) evaluates to f(a, x, 0) f(b, x, 1) f(c, x, 2)
+ */
+#define MAP_UD_I(f, userdata, ...) EVAL(MAP1_UD_I(f, userdata, 0, __VA_ARGS__, ()()(), ()()(), ()()(), 0))
+
+/**
+ * Applies the function macro `f` to each of the remaining parameters, inserts commas between the results,
+ * passes userdata as the second parameter to each invocation, and the index of the invocation as the third parameter,
+ * e.g. MAP_LIST_UD_I(f, x, a, b, c) evaluates to f(a, x, 0), f(b, x, 1), f(c, x, 2)
+ */
+#define MAP_LIST_UD_I(f, userdata, ...) EVAL(MAP_LIST1_UD_I(f, userdata, 0, __VA_ARGS__, ()()(), ()()(), ()()(), 0))
+
+/*
+ * Because the preprocessor can't do arithmetic that produces integer literals for the *_I macros, we have to do it manually.
+ * Since the number of parameters is limited anyways, this is sufficient for all cases. If extra EVAL layers are added, these
+ * definitions have to be extended. This is equivalent to the way Boost.preprocessor does it:
+ * https://github.com/boostorg/preprocessor/blob/develop/include/boost/preprocessor/arithmetic/inc.hpp
+ * The *_I macros could alternatively pass C expressions such as (0), (0+1), (0+1+1...) to the user macro, but passing 0, 1, 2 ...
+ * allows the user to incorporate the index into C identifiers, e.g. to define a function like test_##index () for each
+ * macro invocation.
+ */
+#define MAP_INC_0 1
+#define MAP_INC_1 2
+#define MAP_INC_2 3
+#define MAP_INC_3 4
+#define MAP_INC_4 5
+#define MAP_INC_5 6
+#define MAP_INC_6 7
+#define MAP_INC_7 8
+#define MAP_INC_8 9
+#define MAP_INC_9 10
+#define MAP_INC_10 11
+#define MAP_INC_11 12
+#define MAP_INC_12 13
+#define MAP_INC_13 14
+#define MAP_INC_14 15
+#define MAP_INC_15 16
+#define MAP_INC_16 17
+#define MAP_INC_17 18
+#define MAP_INC_18 19
+#define MAP_INC_19 20
+#define MAP_INC_20 21
+#define MAP_INC_21 22
+#define MAP_INC_22 23
+#define MAP_INC_23 24
+#define MAP_INC_24 25
+#define MAP_INC_25 26
+#define MAP_INC_26 27
+#define MAP_INC_27 28
+#define MAP_INC_28 29
+#define MAP_INC_29 30
+#define MAP_INC_30 31
+#define MAP_INC_31 32
+#define MAP_INC_32 33
+#define MAP_INC_33 34
+#define MAP_INC_34 35
+#define MAP_INC_35 36
+#define MAP_INC_36 37
+#define MAP_INC_37 38
+#define MAP_INC_38 39
+#define MAP_INC_39 40
+#define MAP_INC_40 41
+#define MAP_INC_41 42
+#define MAP_INC_42 43
+#define MAP_INC_43 44
+#define MAP_INC_44 45
+#define MAP_INC_45 46
+#define MAP_INC_46 47
+#define MAP_INC_47 48
+#define MAP_INC_48 49
+#define MAP_INC_49 50
+#define MAP_INC_50 51
+#define MAP_INC_51 52
+#define MAP_INC_52 53
+#define MAP_INC_53 54
+#define MAP_INC_54 55
+#define MAP_INC_55 56
+#define MAP_INC_56 57
+#define MAP_INC_57 58
+#define MAP_INC_58 59
+#define MAP_INC_59 60
+#define MAP_INC_60 61
+#define MAP_INC_61 62
+#define MAP_INC_62 63
+#define MAP_INC_63 64
+#define MAP_INC_64 65
+#define MAP_INC_65 66
+#define MAP_INC_66 67
+#define MAP_INC_67 68
+#define MAP_INC_68 69
+#define MAP_INC_69 70
+#define MAP_INC_70 71
+#define MAP_INC_71 72
+#define MAP_INC_72 73
+#define MAP_INC_73 74
+#define MAP_INC_74 75
+#define MAP_INC_75 76
+#define MAP_INC_76 77
+#define MAP_INC_77 78
+#define MAP_INC_78 79
+#define MAP_INC_79 80
+#define MAP_INC_80 81
+#define MAP_INC_81 82
+#define MAP_INC_82 83
+#define MAP_INC_83 84
+#define MAP_INC_84 85
+#define MAP_INC_85 86
+#define MAP_INC_86 87
+#define MAP_INC_87 88
+#define MAP_INC_88 89
+#define MAP_INC_89 90
+#define MAP_INC_90 91
+#define MAP_INC_91 92
+#define MAP_INC_92 93
+#define MAP_INC_93 94
+#define MAP_INC_94 95
+#define MAP_INC_95 96
+#define MAP_INC_96 97
+#define MAP_INC_97 98
+#define MAP_INC_98 99
+#define MAP_INC_99 100
+#define MAP_INC_100 101
+#define MAP_INC_101 102
+#define MAP_INC_102 103
+#define MAP_INC_103 104
+#define MAP_INC_104 105
+#define MAP_INC_105 106
+#define MAP_INC_106 107
+#define MAP_INC_107 108
+#define MAP_INC_108 109
+#define MAP_INC_109 110
+#define MAP_INC_110 111
+#define MAP_INC_111 112
+#define MAP_INC_112 113
+#define MAP_INC_113 114
+#define MAP_INC_114 115
+#define MAP_INC_115 116
+#define MAP_INC_116 117
+#define MAP_INC_117 118
+#define MAP_INC_118 119
+#define MAP_INC_119 120
+#define MAP_INC_120 121
+#define MAP_INC_121 122
+#define MAP_INC_122 123
+#define MAP_INC_123 124
+#define MAP_INC_124 125
+#define MAP_INC_125 126
+#define MAP_INC_126 127
+#define MAP_INC_127 128
+#define MAP_INC_128 129
+#define MAP_INC_129 130
+#define MAP_INC_130 131
+#define MAP_INC_131 132
+#define MAP_INC_132 133
+#define MAP_INC_133 134
+#define MAP_INC_134 135
+#define MAP_INC_135 136
+#define MAP_INC_136 137
+#define MAP_INC_137 138
+#define MAP_INC_138 139
+#define MAP_INC_139 140
+#define MAP_INC_140 141
+#define MAP_INC_141 142
+#define MAP_INC_142 143
+#define MAP_INC_143 144
+#define MAP_INC_144 145
+#define MAP_INC_145 146
+#define MAP_INC_146 147
+#define MAP_INC_147 148
+#define MAP_INC_148 149
+#define MAP_INC_149 150
+#define MAP_INC_150 151
+#define MAP_INC_151 152
+#define MAP_INC_152 153
+#define MAP_INC_153 154
+#define MAP_INC_154 155
+#define MAP_INC_155 156
+#define MAP_INC_156 157
+#define MAP_INC_157 158
+#define MAP_INC_158 159
+#define MAP_INC_159 160
+#define MAP_INC_160 161
+#define MAP_INC_161 162
+#define MAP_INC_162 163
+#define MAP_INC_163 164
+#define MAP_INC_164 165
+#define MAP_INC_165 166
+#define MAP_INC_166 167
+#define MAP_INC_167 168
+#define MAP_INC_168 169
+#define MAP_INC_169 170
+#define MAP_INC_170 171
+#define MAP_INC_171 172
+#define MAP_INC_172 173
+#define MAP_INC_173 174
+#define MAP_INC_174 175
+#define MAP_INC_175 176
+#define MAP_INC_176 177
+#define MAP_INC_177 178
+#define MAP_INC_178 179
+#define MAP_INC_179 180
+#define MAP_INC_180 181
+#define MAP_INC_181 182
+#define MAP_INC_182 183
+#define MAP_INC_183 184
+#define MAP_INC_184 185
+#define MAP_INC_185 186
+#define MAP_INC_186 187
+#define MAP_INC_187 188
+#define MAP_INC_188 189
+#define MAP_INC_189 190
+#define MAP_INC_190 191
+#define MAP_INC_191 192
+#define MAP_INC_192 193
+#define MAP_INC_193 194
+#define MAP_INC_194 195
+#define MAP_INC_195 196
+#define MAP_INC_196 197
+#define MAP_INC_197 198
+#define MAP_INC_198 199
+#define MAP_INC_199 200
+#define MAP_INC_200 201
+#define MAP_INC_201 202
+#define MAP_INC_202 203
+#define MAP_INC_203 204
+#define MAP_INC_204 205
+#define MAP_INC_205 206
+#define MAP_INC_206 207
+#define MAP_INC_207 208
+#define MAP_INC_208 209
+#define MAP_INC_209 210
+#define MAP_INC_210 211
+#define MAP_INC_211 212
+#define MAP_INC_212 213
+#define MAP_INC_213 214
+#define MAP_INC_214 215
+#define MAP_INC_215 216
+#define MAP_INC_216 217
+#define MAP_INC_217 218
+#define MAP_INC_218 219
+#define MAP_INC_219 220
+#define MAP_INC_220 221
+#define MAP_INC_221 222
+#define MAP_INC_222 223
+#define MAP_INC_223 224
+#define MAP_INC_224 225
+#define MAP_INC_225 226
+#define MAP_INC_226 227
+#define MAP_INC_227 228
+#define MAP_INC_228 229
+#define MAP_INC_229 230
+#define MAP_INC_230 231
+#define MAP_INC_231 232
+#define MAP_INC_232 233
+#define MAP_INC_233 234
+#define MAP_INC_234 235
+#define MAP_INC_235 236
+#define MAP_INC_236 237
+#define MAP_INC_237 238
+#define MAP_INC_238 239
+#define MAP_INC_239 240
+#define MAP_INC_240 241
+#define MAP_INC_241 242
+#define MAP_INC_242 243
+#define MAP_INC_243 244
+#define MAP_INC_244 245
+#define MAP_INC_245 246
+#define MAP_INC_246 247
+#define MAP_INC_247 248
+#define MAP_INC_248 249
+#define MAP_INC_249 250
+#define MAP_INC_250 251
+#define MAP_INC_251 252
+#define MAP_INC_252 253
+#define MAP_INC_253 254
+#define MAP_INC_254 255
+#define MAP_INC_255 256
+#define MAP_INC_256 257
+#define MAP_INC_257 258
+#define MAP_INC_258 259
+#define MAP_INC_259 260
+#define MAP_INC_260 261
+#define MAP_INC_261 262
+#define MAP_INC_262 263
+#define MAP_INC_263 264
+#define MAP_INC_264 265
+#define MAP_INC_265 266
+#define MAP_INC_266 267
+#define MAP_INC_267 268
+#define MAP_INC_268 269
+#define MAP_INC_269 270
+#define MAP_INC_270 271
+#define MAP_INC_271 272
+#define MAP_INC_272 273
+#define MAP_INC_273 274
+#define MAP_INC_274 275
+#define MAP_INC_275 276
+#define MAP_INC_276 277
+#define MAP_INC_277 278
+#define MAP_INC_278 279
+#define MAP_INC_279 280
+#define MAP_INC_280 281
+#define MAP_INC_281 282
+#define MAP_INC_282 283
+#define MAP_INC_283 284
+#define MAP_INC_284 285
+#define MAP_INC_285 286
+#define MAP_INC_286 287
+#define MAP_INC_287 288
+#define MAP_INC_288 289
+#define MAP_INC_289 290
+#define MAP_INC_290 291
+#define MAP_INC_291 292
+#define MAP_INC_292 293
+#define MAP_INC_293 294
+#define MAP_INC_294 295
+#define MAP_INC_295 296
+#define MAP_INC_296 297
+#define MAP_INC_297 298
+#define MAP_INC_298 299
+#define MAP_INC_299 300
+#define MAP_INC_300 301
+#define MAP_INC_301 302
+#define MAP_INC_302 303
+#define MAP_INC_303 304
+#define MAP_INC_304 305
+#define MAP_INC_305 306
+#define MAP_INC_306 307
+#define MAP_INC_307 308
+#define MAP_INC_308 309
+#define MAP_INC_309 310
+#define MAP_INC_310 311
+#define MAP_INC_311 312
+#define MAP_INC_312 313
+#define MAP_INC_313 314
+#define MAP_INC_314 315
+#define MAP_INC_315 316
+#define MAP_INC_316 317
+#define MAP_INC_317 318
+#define MAP_INC_318 319
+#define MAP_INC_319 320
+#define MAP_INC_320 321
+#define MAP_INC_321 322
+#define MAP_INC_322 323
+#define MAP_INC_323 324
+#define MAP_INC_324 325
+#define MAP_INC_325 326
+#define MAP_INC_326 327
+#define MAP_INC_327 328
+#define MAP_INC_328 329
+#define MAP_INC_329 330
+#define MAP_INC_330 331
+#define MAP_INC_331 332
+#define MAP_INC_332 333
+#define MAP_INC_333 334
+#define MAP_INC_334 335
+#define MAP_INC_335 336
+#define MAP_INC_336 337
+#define MAP_INC_337 338
+#define MAP_INC_338 339
+#define MAP_INC_339 340
+#define MAP_INC_340 341
+#define MAP_INC_341 342
+#define MAP_INC_342 343
+#define MAP_INC_343 344
+#define MAP_INC_344 345
+#define MAP_INC_345 346
+#define MAP_INC_346 347
+#define MAP_INC_347 348
+#define MAP_INC_348 349
+#define MAP_INC_349 350
+#define MAP_INC_350 351
+#define MAP_INC_351 352
+#define MAP_INC_352 353
+#define MAP_INC_353 354
+#define MAP_INC_354 355
+#define MAP_INC_355 356
+#define MAP_INC_356 357
+#define MAP_INC_357 358
+#define MAP_INC_358 359
+#define MAP_INC_359 360
+#define MAP_INC_360 361
+#define MAP_INC_361 362
+#define MAP_INC_362 363
+#define MAP_INC_363 364
+#define MAP_INC_364 365
+#define MAP_INC_365 366
+
+
 #endif