Add State/StateT #302
Add State/StateT #302
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To avoid stack overflows, a couple design choices were made that make State more complicated than it seems like it should be. State wraps F[S => F[(S, A)]] instead of the more familiar S => F[(S, A)]. This allows for effective trampolining. State[S, A] is a type alias for StateT[Trampoline, S, A] to avoid stack overflows that are easy to hit if Id is used instead of Trampoline. State is currently in its own module because it depends on Free (for trampolining) but doesn't seem to belong in the Free module. Since it's tough to avoid trampolining in FP Scala, we may want to reconsider whether Free should be in core. There is a brief ScalaDoc comment on StateT, but a proper markdown file should probably be created. If this gets merged, I can send a follow-up PR with some more documentation.
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This is embarrassing. One of my more recent changes appears to have made the test not compile and I guess I didn't run the tests again before submitting. Will fix :( |
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LGTM, plus I like the runX names. |
| /** | ||
| * Run with `S`'s empty monoid value as the initial state. | ||
| */ | ||
| def runEmpty(implicit S: Monoid[S], F: FlatMap[F]) = run(S.empty) |
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This is missing the return type.
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Good catch. Pushed a new commit that fixes the three instances.
And also simplify the StateT.map implementation.
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Looks great (application of the state function now cannot be done recursively without passing through |
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Looks great to me. 👍 |
| lazy val state = project.dependsOn(macros, core, free, tests % "test -> test") | ||
| .settings(moduleName := "cats-state") | ||
| .settings(catsSettings) | ||
| .settings(noPublishSettings) |
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Is there a reason not to publish this?
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Oh, good catch. I would be fine publishing it.
Want to submit a small PR fixing this?
Fixes #122.
To avoid stack overflows, a couple design choices were made that make
State more complicated than it seems like it should be.
State wraps F[S => F[(S, A)]] instead of the more familiar S => F[(S, A)]. This allows for effective trampolining.
State[S, A] is a type alias for StateT[Trampoline, S, A] to avoid stack
overflows that are easy to hit if Id is used instead of Trampoline.
State is currently in its own module because it depends on Free (for
trampolining) but doesn't seem to belong in the Free module. Since it's
tough to avoid trampolining in FP Scala, we may want to reconsider
whether Free should be in core.
There is a brief ScalaDoc comment on StateT, but a proper markdown file
should probably be created. If this gets merged, I can send a follow-up
PR with some more documentation.