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| 1 | +Contains exercises on implementing custom transducers in Clojure. |
| 2 | + |
| 3 | +##Fair notice |
| 4 | + |
| 5 | +Implementing transducers is hard and you may feel confused. |
| 6 | + |
| 7 | + |
| 8 | + |
| 9 | +That's normal and to be expected, it happens also to me when I read my own code. Just don't give up. |
| 10 | + |
| 11 | +##No Operation transducer |
| 12 | + |
| 13 | +Implement a transducer that does nothing but passing the data from his input to his output. Functionally speaking, it is an identity transducer. |
| 14 | + |
| 15 | +```clojure |
| 16 | +(into [] nop (range3)) |
| 17 | +; => [0 1 2] |
| 18 | +``` |
| 19 | + |
| 20 | +Use it as your transducer template for the following. |
| 21 | + |
| 22 | +##Prepare for battle |
| 23 | + |
| 24 | +* Implement the`(debug in out)` transducer that helps to debug. |
| 25 | + |
| 26 | +```clojure |
| 27 | +; We use this function instead of `into` for debugging. |
| 28 | +; The reason is that this avoids using transient |
| 29 | +; structures which do not `print` nicely. |
| 30 | +(defnslow-into [to xf from] |
| 31 | + (transduce xf conj to from)) |
| 32 | + |
| 33 | +(slow-into [] (debug"in""out") (range3)) |
| 34 | +;; Outputs: |
| 35 | +; in 0 |
| 36 | +; out [0] |
| 37 | +; in 1 |
| 38 | +; out [0 1] |
| 39 | +; in 2 |
| 40 | +; out [0 1 2] |
| 41 | +``` |
| 42 | + |
| 43 | +* Add the`(debug)`,`(debug indent)` and`(debug indent in out)` variants for convenience. They are all calling the`(debug in out)` transducer under the hood. |
| 44 | + |
| 45 | +```clojure |
| 46 | +(slow-into [] |
| 47 | + (comp (debug) |
| 48 | + (debug2) |
| 49 | + (debug4">""<")) |
| 50 | + (debug" >"" <")); 6-spaces prefix |
| 51 | + (range3)) |
| 52 | +;; Outputs: |
| 53 | +; > 0 |
| 54 | +; > 0 |
| 55 | +; > 0 |
| 56 | +; > 0 |
| 57 | +; < [0] |
| 58 | +; < [0] |
| 59 | +; < [0] |
| 60 | +; < [0] |
| 61 | +; > 1 |
| 62 | +; > 1 |
| 63 | +; > 1 |
| 64 | +; > 1 |
| 65 | +; < [0 1] |
| 66 | +; < [0 1] |
| 67 | +; < [0 1] |
| 68 | +; < [0 1] |
| 69 | +; > 2 |
| 70 | +; > 2 |
| 71 | +; > 2 |
| 72 | +; > 2 |
| 73 | +; < [0 1 2] |
| 74 | +; < [0 1 2] |
| 75 | +; < [0 1 2] |
| 76 | +; < [0 1 2] |
| 77 | +``` |
| 78 | + |
| 79 | +Strange patterns, it reminds me of ... |
| 80 | + |
| 81 | +**The PHP Hadouken !!!** |
| 82 | + |
| 83 | + |
| 84 | + |
| 85 | +We are now ready to face real transducer implementations and confront an army of problems. |
| 86 | + |
| 87 | +##May I beg your pardon? |
| 88 | + |
| 89 | +You heard me well, I want you to implement the following transducer. |
| 90 | + |
| 91 | +```clojure |
| 92 | +(defbeg-data (list:may:i:beg:your:pardon:?)) |
| 93 | + |
| 94 | +(into [] (beg2) beg-data) |
| 95 | +; => [:may :may :i :i :beg :beg :your :your :pardon :pardon :? :?] |
| 96 | +``` |
| 97 | + |
| 98 | +Make sure that you are handling the early termination as well. |
| 99 | + |
| 100 | +```clojure |
| 101 | +(into [] |
| 102 | + (comp (take3) |
| 103 | + (beg2)) |
| 104 | + beg-data) |
| 105 | +; => [:may :may :i :i :beg :beg] |
| 106 | +``` |
| 107 | + |
| 108 | +Test for both sides. |
| 109 | + |
| 110 | +```clojure |
| 111 | +(into [] |
| 112 | + (comp (beg2) |
| 113 | + (take3)) |
| 114 | + beg-data) |
| 115 | +; => [:may :may :i] |
| 116 | +``` |
| 117 | + |
| 118 | +Test with the debug transducer (expect problems and losing some hair). |
| 119 | + |
| 120 | +```clojure |
| 121 | +(slow-into [] |
| 122 | + (comp (debug0) |
| 123 | + (beg2) |
| 124 | + (debug2) |
| 125 | + (take3) |
| 126 | + (debug4)) |
| 127 | + beg-data) |
| 128 | +; Output: |
| 129 | +; > :may |
| 130 | +; > :may |
| 131 | +; > :may |
| 132 | +; < [:may] |
| 133 | +; < [:may] |
| 134 | +; > :may |
| 135 | +; > :may |
| 136 | +; < [:may :may] |
| 137 | +; < [:may :may] |
| 138 | +; < [:may :may] |
| 139 | +; > :i |
| 140 | +; > :i |
| 141 | +; > :i |
| 142 | +; < [:may :may :i] |
| 143 | +; < #reduced[{:status :ready, :val [:may :may :i]} 0x8cdcdd1] |
| 144 | +; < #reduced[{:status :ready, :val [:may :may :i]} 0x8cdcdd1] |
| 145 | + |
| 146 | +; Result: |
| 147 | +; => [:may :may :i] |
| 148 | +``` |
| 149 | + |
| 150 | +The`beg` transducer should not continue sending data downstream after it receives a reduced result. Fix your implementation if needed. |
| 151 | + |
| 152 | +##All your data are belong to me |
| 153 | + |
| 154 | + |
| 155 | + |
| 156 | +Implement the`my-cat` transducer. For each step, you will need to adapt your implementation to the new requirements described by the test samples. |
| 157 | + |
| 158 | +* Step 1, shapeless cat |
| 159 | + |
| 160 | +It functions similarly to`clojure.core/cat`. Don't handle early termination at the moment. |
| 161 | + |
| 162 | +```clojure |
| 163 | +(defcat-data [[12:fish3] [:heat4] [5:sleep6] [7]]) |
| 164 | + |
| 165 | +(into [] my-cat cat-data) |
| 166 | +; => [1 2 :fish 3 :heat 4 5 :sleep 6 7] |
| 167 | +``` |
| 168 | + |
| 169 | +* Step 2, hungry cat |
| 170 | + |
| 171 | +As you can see, the transducer is keeping for itself all the fishes and the heat. |
| 172 | + |
| 173 | +```clojure |
| 174 | +(into [] my-cat cat-data) |
| 175 | +; => [1 2 3 4 5 :sleep 6 7] |
| 176 | +``` |
| 177 | + |
| 178 | +* Step 3, sleepy cat |
| 179 | + |
| 180 | +That version of the transducer falls asleep an the`:sleep` keyword and do not process any subsequent data. |
| 181 | + |
| 182 | +```clojure |
| 183 | +(into [] my-cat cat-data) |
| 184 | +; => [1 2 3 4 5] |
| 185 | +``` |
| 186 | + |
| 187 | +* Step 4, correct cat |
| 188 | + |
| 189 | +At last, we want our transducer to respect the normal early termination (with`reduced?` tested on the downstream result) in a correct manner. |
| 190 | + |
| 191 | +```clojure |
| 192 | +(into [] (comp (take2) |
| 193 | + my-cat) |
| 194 | + cat-data) |
| 195 | +; => [1 2 3 4] |
| 196 | + |
| 197 | +(into [] (comp my-cat |
| 198 | + (take2)) |
| 199 | + cat-data) |
| 200 | +; => [1 2] |
| 201 | + |
| 202 | +(slow-into [] (comp (debug0) |
| 203 | + my-cat; try replacing it with `cat` and compare |
| 204 | + (debug2) |
| 205 | + (take2) |
| 206 | + (debug4)) |
| 207 | + cat-data) |
| 208 | +;; Outputs: |
| 209 | +; > [1 2 :fish 3] |
| 210 | +; > 1 |
| 211 | +; > 1 |
| 212 | +; < [1] |
| 213 | +; < [1] |
| 214 | +; > 2 |
| 215 | +; > 2 |
| 216 | +; < [1 2] |
| 217 | +; < #reduced[{:status :ready, :val [1 2]} 0x517a7e8e] |
| 218 | +; < #reduced[{:status :ready, :val [1 2]} 0x517a7e8e] |
| 219 | +``` |
| 220 | + |
| 221 | +* Provide an idiomatic equivalent to`my-cat`. |
| 222 | + |
| 223 | + |
| 224 | +##A.D.D. transducer |
| 225 | + |
| 226 | +* Implement a transducer that daydream during a number of elements. While in the daydream state, it buffers its input. When it stops daydreaming, it processes all of its buffer as a batch, then daydreams again. |
| 227 | + |
| 228 | +The`a-d-d` transducer never loses data. |
| 229 | + |
| 230 | +```clojure |
| 231 | +(into [] (a-d-d3) (range10)) |
| 232 | +; => [0 1 2 3 4 5 6 7 8 9] |
| 233 | + |
| 234 | +; Try: |
| 235 | +(slow-into [] (comp (debug0) |
| 236 | + (a-d-d3) |
| 237 | + (debug2)) |
| 238 | + (range10)) |
| 239 | +``` |
| 240 | + |
| 241 | +* Change`a-d-d` so that it works similarly to`clojure.core/partition-all`. |
| 242 | + |
| 243 | +```clojure |
| 244 | +(into [] (a-d-d3) (range10)) |
| 245 | +; => [[0 1 2] [3 4 5] [6 7 8] [9]] |
| 246 | +``` |
| 247 | + |
| 248 | +* Verify that it works well with early termination. |
| 249 | + |
| 250 | +```clojure |
| 251 | +(slow-into [] (comp (debug0) |
| 252 | + (a-d-d3) |
| 253 | + (debug2) |
| 254 | + (take2)) |
| 255 | + (range10)) |
| 256 | +; => [[0 1 2] [3 4 5]] |
| 257 | +``` |
| 258 | + |
| 259 | +##A reducer inside a transducer |
| 260 | + |
| 261 | +Implement`serieduce` which provides a transducer which reduces incoming elements and emits all intermediary elements. |
| 262 | + |
| 263 | +```clojure |
| 264 | +(into [] (serieduce conj [12]) (range36)) |
| 265 | +; => [[1 2 3] [1 2 3 4] [1 2 3 4 5]] |
| 266 | + |
| 267 | +(into [] (serieduce +) (range5)) |
| 268 | +; => [0 1 3 6 10] |
| 269 | +``` |
| 270 | + |
| 271 | +##Congratulations if you made it that far! |
| 272 | + |
| 273 | + |
| 274 | + |
| 275 | +You are one of a few. |
