-
Notifications
You must be signed in to change notification settings - Fork 516
/
IOLocal.scala
236 lines (216 loc) · 8.57 KB
/
IOLocal.scala
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
/*
* Copyright 2020-2022 Typelevel
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package cats.effect
/**
* [[IOLocal]] provides a handy way of manipulating a context on different scopes.
*
* In some scenarios, [[IOLocal]] can be considered as an alternative to [[cats.data.Kleisli]].
*
* [[IOLocal]] should not be treated as [[cats.effect.kernel.Ref Ref]], since the former abides
* different laws.
*
* Once a fiber is forked, for example by `Spawn[F].start`, the forked fiber manipulates the
* copy of the parent's context. For example, two forked fibers will never see each other's
* modifications to the same [[IOLocal]], each fiber will only see its own modifications.
*
* ===Operations on [[IOLocal]] are visible to the fiber===
*
* {{{
* ┌────────────┐ ┌────────────┐ ┌────────────┐
* │ Fiber A │ update(_ + 1) │ Fiber A │ update(_ + 1) │ Fiber A │
* │ (local 42) │──────────────►│ (local 43) │──────────────►│ (local 44) │
* └────────────┘ └────────────┘ └────────────┘
* }}}
*
* {{{
* def inc(name: String, local: IOLocal[Int]): IO[Unit] =
* local.update(_ + 1) >> local.get.flatMap(current => IO.println(s"fiber $$name: $$current"))
*
* for {
* local <- IOLocal(42)
* _ <- inc(1, local)
* _ <- inc(2, local)
* current <- local.get
* _ <- IO.println(s"fiber A: $$current")
* } yield ()
*
* // output:
* // update 1: 43
* // update 2: 44
* // fiber A: 44
* }}}
*
* ===A forked fiber operates on a copy of the parent [[IOLocal]]===
*
* A '''forked''' fiber (i.e. via `Spawn[F].start`) operates on a '''copy''' of the parent
* `IOLocal`. Hence, the children operations are not reflected on the parent context.
*
* {{{
* ┌────────────┐ ┌────────────┐
* fork │ Fiber B │ update(_ - 1) │ Fiber B │
* ┌─────►│ (local 42) │──────────────►│ (local 41) │
* │ └────────────┘ └────────────┘
* ┌────────────┐─┘ ┌────────────┐
* │ Fiber A │ │ Fiber A │
* │ (local 42) │────────────────────────────────────►│ (local 42) │
* └────────────┘─┐ └────────────┘
* │ ┌────────────┐ ┌────────────┐
* │ fork │ Fiber C │ update(_ + 1) │ Fiber C │
* └─────►│ (local 42) │──────────────►│ (local 43) │
* └────────────┘ └────────────┘
* }}}
*
* {{{
* def update(name: String, local: IOLocal[Int], f: Int => Int): IO[Unit] =
* local.update(f) >> local.get.flatMap(current => IO.println(s"$$name: $$current"))
*
* for {
* local <- IOLocal(42)
* fiber1 <- update("fiber B", local, _ - 1).start
* fiber2 <- update("fiber C", local, _ + 1).start
* _ <- fiber1.joinWithNever
* _ <- fiber2.joinWithNever
* current <- local.get
* _ <- IO.println(s"fiber A: $$current")
* } yield ()
*
* // output:
* // fiber B: 41
* // fiber C: 43
* // fiber A: 42
* }}}
*
* ===Parent operations on [[IOLocal]] are invisible to children===
*
* {{{
* ┌────────────┐ ┌────────────┐
* fork │ Fiber B │ update(_ + 1) │ Fiber B │
* ┌─────►│ (local 42) │──────────────►│ (local 43) │
* │ └────────────┘ └────────────┘
* ┌────────────┐─┘ ┌────────────┐
* │ Fiber A │ update(_ - 1) │ Fiber A │
* │ (local 42) │────────────────────────────────────►│ (local 41) │
* └────────────┘─┐ └────────────┘
* │ ┌────────────┐ ┌────────────┐
* │ fork │ Fiber C │ update(_ + 2) │ Fiber C │
* └─────►│ (local 42) │──────────────►│ (local 44) │
* └────────────┘ └────────────┘
* }}}
*
* {{{
* def update(name: String, local: IOLocal[Int], f: Int => Int): IO[Unit] =
* IO.sleep(1.second) >> local.update(f) >> local.get.flatMap(current => IO.println(s"$$name: $$current"))
*
* for {
* local <- IOLocal(42)
* fiber1 <- update("fiber B", local, _ + 1).start
* fiber2 <- update("fiber C", local, _ + 2).start
* _ <- fiber1.joinWithNever
* _ <- fiber2.joinWithNever
* _ <- update("fiber A", local, _ - 1)
* } yield ()
*
* // output:
* // fiber B: 43
* // fiber C: 44
* // fiber A: 41
* }}}
*
* @tparam A
* the type of the local value
*/
sealed trait IOLocal[A] {
/**
* Returns the current value.
*/
def get: IO[A]
/**
* Sets the current value to `value`.
*/
def set(value: A): IO[Unit]
/**
* Replaces the current value with the initial value.
*/
def reset: IO[Unit]
/**
* Modifies the current value using the given update function.
*/
def update(f: A => A): IO[Unit]
/**
* Like [[update]] but allows the update function to return an output value of type `B`.
*
* @see
* [[update]]
*/
def modify[B](f: A => (A, B)): IO[B]
/**
* Replaces the current value with `value`, returning the previous value.
*
* The combination of [[get]] and [[set]].
*
* @see
* [[get]]
* @see
* [[set]]
*/
def getAndSet(value: A): IO[A]
/**
* Replaces the current value with the initial value, returning the previous value.
*
* The combination of [[get]] and [[reset]].
*
* @see
* [[get]]
* @see
* [[reset]]
*/
def getAndReset: IO[A]
}
object IOLocal {
/**
* Creates a new instance of [[IOLocal]] with the given default value.
*
* The creation is effectful, because [[IOLocal]] models mutable state, and allocating mutable
* state is not pure.
*
* @param default
* the default value
* @tparam A
* the type of the local value
*/
def apply[A](default: A): IO[IOLocal[A]] =
IO {
new IOLocal[A] { self =>
override def get: IO[A] =
IO.Local(state => (state, state.get(self).map(_.asInstanceOf[A]).getOrElse(default)))
override def set(value: A): IO[Unit] =
IO.Local(state => (state + (self -> value), ()))
override def reset: IO[Unit] =
IO.Local(state => (state - self, ()))
override def update(f: A => A): IO[Unit] =
get.flatMap(a => set(f(a)))
override def modify[B](f: A => (A, B)): IO[B] =
get.flatMap { a =>
val (a2, b) = f(a)
set(a2).as(b)
}
override def getAndSet(value: A): IO[A] =
get <* set(value)
override def getAndReset: IO[A] =
get <* reset
}
}
}