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nigredo-tori
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Mar 22, 2026
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| * `gen` will make an effort to keep the resulting value's type as narrow as possible, which can be useful for typeclass families. As a | ||
| * contrived example, when configured with `type Typeclass[x] = Either[Any, x]` and corresponding `join` and `split`, `gen` may derive an | ||
| * `Either[String, T]` if the target type and available instances allow it. To take advantage of it, one has to define `join` and `split` | ||
| * in such a way that they propagate narrowed typeclasses as appropriate: | ||
| * | ||
| * {{{ | ||
| * def join[L, T](caseClass: CaseClass[Either[L, *], T]): Either[L, T] | ||
| * }}} |
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| val prefixType = c.prefix.tree.tpe | ||
| val prefixObject = prefixType.typeSymbol | ||
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| val TypeClassNme = TypeName("Typeclass") | ||
| val typeDefs = prefixType.baseClasses.flatMap { baseClass => | ||
| baseClass.asType.toType.decls.collectFirst { | ||
| case tpe: TypeSymbol if tpe.name == TypeClassNme => | ||
| tpe.toType.asSeenFrom(prefixType, baseClass) | ||
| } | ||
| } | ||
| val typeclass = typeDefs.headOption.fold( | ||
| c.abort(c.enclosingPosition, s"the derivation $prefixObject does not define the Typeclass type constructor") | ||
| )(_.typeConstructor) | ||
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| TypeConstructor(typeclass, appliedType(typeclass, _)) |
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This was previously done in gen.
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| s"""expected a * -> * HKT, | ||
| |got: $tpe as ${showRaw(tpe)} | ||
| |eta-expanded: ${tpe.etaExpand} as ${showRaw(tpe.etaExpand)}""" |
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This is probably too verbose, but I feel like the more information we get here the better. The end users probably wouldn't see this error.
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| import magnolia1._ | ||
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| // Slimmed down version of schema derivation from the Caliban library. |
Author
| assert(error contains """ | ||
| |magnolia: could not find Show.Typeclass for type Double | ||
| assert(clue(error) contains """ | ||
| |magnolia: could not find magnolia1.examples.Show.Typeclass[Double] |
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The error messages were changed to fully show the required type.
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| implicit def genNarrow[Tc[_] <: CollectFields[Any, _], A]: Tc[A] = | ||
| macro Magnolia.genNarrow[Tc, A] |
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I'd like to be able to write
implicit def genNarrow[Out, A]: CollectFields[Out, A] =
macro Magnolia.genNarrow[CollectFields[Out, *], A]However, for whatever reason, this seems to lose the information about what Out actually is.
Wrapping the whole thing in another macro (as in the schema example) seems to solve this, but it's a weird magical workaround for what looks like a compiler bug.
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See #627.
It's difficult to narrow recursive instances, especially in Scala 2 where we don't constrain
joinandsplitsignatures. The most robust solution (although not the most convenient) is to give the users a way to manually specify the desired typeclass.This PR adds a new macro,
genNarrow, that does just that. It reuses most of the implementation ofgen, but replaces uses ofTypeclasswith a specified* -> *HKT.