Improvements to Typeclass Derivation #5839
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Based on a bunch of other in flight PRs in order to avoid rebases once these are in. |
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Only last two commits are new. |
1. add a Generic instance only if the implementation of a derived
typeclass mentions the type.
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| @@ -210,14 +212,33 @@ trait Deriving { this: Typer => | |||
| addDerivedInstance(defn.GenericType.name, genericCompleter, codePos, reportErrors = false) | |||
| } | |||
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| /** If any of the instances has a companion with a `derived` member | |||
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docs/reference/derivation.md should be update to reflect that change. I think it would make sense to keep this under specified and say that Generic instance are only guaranteed to be cached when writing derives Generic.
| @@ -161,33 +161,67 @@ trait Deriving { this: Typer => | |||
| * | |||
| * implicit def derived$D(implicit ev_1: D[T_1], ..., ev_n: D[T_n]): D[C[Ts]] = D.derived | |||
| * | |||
| * See test run/typeclass-derivation2 for examples that spell out what would be generated. | |||
| * Note that the name of the derived method containd the name in the derives clause, not | |||
| * See the body of this method for who to generalize this to typeclasses with more | |||
| if (nparams == 0) Nil | ||
| else if (nparams == 1) tparam :: Nil | ||
| else typeClass.typeParams.map(tcparam => | ||
| tparam.copy(name = s"${tparam.name}_${tcparam.name}".toTypeName) |
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What happens if this generates two identical names?
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How could that happen? Both parameter lists use disjoint names.
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class Foo[A_B, A]class Bar[_B_C, _C]
yields
A_B_B_CA_B_CA_B_CA_C
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Good point. We should use a reserved separator then. Maybe _$_?
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I'll do the change in the follow-up PR #5843
Add a Generic instance only if the implementation of a derived
typeclass mentions the type.
Allow derivations for typeclasses with more or less type parameters than 1.