Proposal: allow automatic derivation of HFunctor instances. #170
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Generally speaking, the majority of effects do not require scoping,
and as a result we encounter often this definition of `HFunctor`:
```haskell
instance HFunctor MyEffect where
hmap _ = coerce
```
This definition is valid as long as there are no occurrences of a
higher-order (i.e. scoped) variable in the effect. The `Coercible`
typeclass allows us to express this in the type system:
```haskell
default hmap :: Coercible (h m a) (h n a)
=> (forall x . m x -> n x)
-> (h m a -> h n a)
hmap _ = coerce
```
By providing this as a valid default implementation of `hmap`, and
enabling `DeriveAnyClass`/`DerivingStrategies` in effect definitions,
we save cruft and boilerplate without any impact to backwards
compatibility or performance.
|
I should mention that this was inspired by Polysemy’s implementation of effect handling. Polysemy also provides a default implementation of |
@patrickt: see also #54 for a definition of |
| => (forall x . m x -> n x) | ||
| -> (h m a -> h n a) | ||
| hmap _ = coerce | ||
| {-# INLINE hmap #-} |
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What do the error messages look like when you try to use the default definition with a higher-order effect?
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It could be worse, but it could be better:
src/Control/Effect/Writer.hs:28:1: error:
• Couldn't match representation of type ‘m’ with that of ‘n’
arising from a use of ‘Control.Effect.Carrier.$dmhmap’
‘m’ is a rigid type variable bound by
the type signature for:
hmap :: forall (m :: * -> *) (n :: * -> *) a.
(forall x. m x -> n x) -> Writer w m a -> Writer w n a
at src/Control/Effect/Writer.hs:28:1-37
‘n’ is a rigid type variable bound by
the type signature for:
hmap :: forall (m :: * -> *) (n :: * -> *) a.
(forall x. m x -> n x) -> Writer w m a -> Writer w n a
at src/Control/Effect/Writer.hs:28:1-37
• In the expression: Control.Effect.Carrier.$dmhmap @(Writer w)
src/Control/Effect/Fail.hs
Outdated
| hmap _ = coerce | ||
| {-# INLINE hmap #-} | ||
| deriving stock Functor | ||
| deriving anyclass HFunctor |
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I guess we could also have done instance HFunctor Fail w/o a where clause, but I must admit I like the DerivingStrategies here.
|
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||
| instance HFunctor Fail where | ||
| hmap _ = coerce | ||
| {-# INLINE hmap #-} |
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Do we lose inlining with the default definition? Or does the INLINE pragma on the default definition apply to each derived instance?
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The INLINE pragma applies to each derived instance 🎉
src/Control/Effect/Carrier.hs
Outdated
| @@ -19,9 +19,9 @@ class HFunctor h where | |||
| {-# INLINE fmap' #-} | |||
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| -- | Higher-order functor map of a natural transformation over higher-order positions within the effect. | |||
| -- A definition for 'hmap' over first-order effects can be derived with the @-XDeriveAnyClass@ extension. | |||
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You don’t actually have to use -XDeriveAnyClass; you can also do instance HFunctor MyEffect w/o a where clause. Might be worth noting that?
Generally speaking, the majority of effects do not require scoping,
and as a result we encounter often this definition of
HFunctor:This definition is valid as long as there are no occurrences of a
higher-order (i.e. scoped) variable in the effect. The
Coercibletypeclass allows us to express this in the type system:
By providing this as a valid default implementation of
hmap, andenabling
DeriveAnyClass/DerivingStrategiesin effect definitions,we save cruft and boilerplate without any impact to backwards
compatibility or performance.
DerivingStrategiesdates to GHC 8.2,so all targeted GHC’s will be fine with this.
Fixes #54.