Capability-retaining zoom and magnify
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This change allows callers of the `Capability.State.zoom` and `Capability.Reader.magnify` functions to pass along a set of capabilities from the current monadic context to the focusing action. Retained capabilities are specified via a typelevel parameter of kind `[(* -> *) -> Constraint]`. Implementation relies on `unsafeCoerce` to convert a dictionary for a wrapped monad instance to the original monad. Fixes #46.
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aspiwack
approved these changes
May 9, 2019
| @"foobar" @"foo" @(Rename 1 :.: Pos 1 "foobar") | ||
| @('[HasState "foobar" (Int,Int)]) $ do | ||
| incFoo | ||
| incFoobar |
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I'm not sure that we should encourage people to have to concurrent HasState instances. It's technically legal, but it may be surprising. I shouldn't prevent us from merging this, but maybe we will want to split this into a two-state example and a zoom-while-preserving-stuff example (with, for instance, a magnifyError while preservinig a state).
aherrmann
approved these changes
May 9, 2019
| -- See <https://github.com/tweag/capability/issues/46>. | ||
| zoom :: forall outertag innertag t outer inner m a. | ||
| ( forall x. Coercible (t m x) (m x) | ||
| zoom :: forall outertag innertag t (cs :: [(* -> *) -> Constraint]) outer inner m a. |
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Same question about aliasing. Maybe it's worth a sentence in the docs.
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@aherrmann Right, I skipped |
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This change allows callers of the
Capability.State.zoomandCapability.Reader.magnifyfunctions to pass along a set of capabilities from the current monadic context to the focusing action. Retained capabilities are specified via a typelevel parameter of kind[(* -> *) -> Constraint].Implementation relies on
unsafeCoerceto convert a dictionary for a wrapped monad instance to the original monad.Fixes #46.