I was working on an existing AutoLISP program for AutoCAD while I found that during “Check Text in Editor” (or “Check Selection”) I got an error. It worked fine to just load and run the program but check did not. Trying to make an application (FAS or VLX) will also give this error.
; error: too many arguments: (PROGN 1 2 3 ... )
It was really confusing until I realized that progn only accepts 255 arguments. Often progn is used to together with if to allow more expressions to be evaluated and in a case like that it can easily happen that there are more than 255 arguments.
This is en example of how to reproduce this:
(progn
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
211 212 213 214 215 216 217 218 219 220 221 222 223 224
225 226 227 228 229 230 231 232 233 234 235 236 237 238
239 240 241 242 243 244 245 246 247 248 249 250 251 252
253 254 255 256
)
The workaround I used was to add a few extra nested progn and problem solved.
Another way is to use cond like this:
(cond (T
1 2 3 all the way to 256 or higher will work fine
))
And of course create a new function with defun.
But what about using (repeat 1 …) as a workaround saving an extra pair of parentheses compared to cond? Unfortunately repeat has the same limitation of 255 expressions as progn.
; error: too many arguments: (REPEAT 1 1 2 ... )
What then is progn other than a way to bundle together a lot of expressions?
Progn evaluates each expression sequentially and returns the value of the last expression. In the progn example above the returned value would be 256.