Support deriving via by kl0tl · Pull Request #3824 · purescript/purescript · GitHub

It looks like it works but there’s a few things I need help with:

I had to add cases for ParensInType in various places. This constructor doesn’t usually seem to be present in types but it is in via types. Is it supposed to be somehow removed?
This is perhaps related to the previous point but the error messages I added contain unfortunate line breaks after prettified via types.
I would like to throw a more helpful error than NoInstanceFound when the kinds of the via type and of the instance head last type don’t unify. I tried to elaborate those kinds but elaborating the kind of an application whose first parameter has a polymorphic kind fails.

For instance Data.Functor.Compose.Compose has such a polymorphic kind:

newtype Compose :: ∀ k1 k2. (k1 -> Type) -> (k2 -> k1) -> k2 -> Type

meaning that elaborating the kind of Compose (Either e) Maybe fails when elaborating the application of Compose to Either e. Should I approach this differently?

Also I took the liberty to rename MissingNewtypeSuperclassInstance to MissingSuperclassInstance since this error is thrown for both newtype and via instances, removed the mention of derived superclass instance from its message (since any instance actually qualify) and replaced the InvalidNewtypeInstance error thrown when trying to derive a newtype instance for a nullary type class by a new CannotDeriveNullaryTypeClassInstance error (which is also thrown for via instances).

Close #3302.

I had to had cases for ParensInType in various places. This constructor doesn’t usually seem to be present in types but it is in via types. Is it supposed to be somehow removed?

ParensInType is removed as part of operator desugaring (removeParens). You are probably missing a case when traversing types for the deriving via declaration.

Oh thank you, I added the guilty case 🙇 Also I fixed my error messages and moved most of the deriving via checks into the type checker because I was duplicating a lot of work to infer kinds.

Since this is a contextual keyword, you will need to make sure to add it ident, qualIdent, and label.

Good catch! Sorry about that, I added a few test cases to cover this.

I managed to ensure the via kind unifies with the kind of the type for which the instance is derived! Elaborating the kinds didn’t work but inferring them after binding all free variables to fresh kinds (like it is done when checking instances declarations) does 🎉

Does this throw away the unsolved constraints?

Oh, but that's fine because of the current SolverOptions, so the solver won't emit unsolved constraints and instead just fail, right? I guess that means we can't write something like

data T

derive via (...) instance blah :: Coercible X Y => Cls T

i.e. defer Coercible constraints in the instance head, but that's not a big loss, and maybe something that can be solved later?

The solver indeed fails when typechecking the instance declaration if the generated constraint can’t be solved but it doesn’t try to solve the instance context, so you can write such instance.

Should f be called on kinds here? There’s no case for the KindsDoNotUnify error for instance so maybe it shouldn’t 🤔

If there are cases that are missing it's probably an oversight.

The following SimpleErrorMessage constructors hold SourceTypes but aren’t matched:

InfiniteKind
EscapedSkolem
KindsDoNotUnify
MissingClassMember
UserDefinedWarning
QuantificationCheckFailureInType
UnsupportedTypeInKind

Should I add cases for them in this PR? Matching every constructor explicitly would be verbose but could help to prevent this in the future.

One thing to consider with this syntax is #1120. If we allowed forall for instances, the via clause would be outside of the binder. Should we discuss making via a proper reserved word and put it at the end of the instance? Obviously that ticket isn't a done deal, but I think it's something other maintainers are keen on, so some bike-shedding might be appropriate.

kl0tl force-pushed the deriving-via branch from 4663371 to 6521017 Compare March 28, 2020 20:31

natefaubion reviewed Mar 28, 2020

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kl0tl force-pushed the deriving-via branch from 6521017 to baba10c Compare March 29, 2020 17:16

kl0tl force-pushed the deriving-via branch 2 times, most recently from 2a3abeb to 25050ef Compare March 29, 2020 18:19

JordanMartinez mentioned this pull request Mar 29, 2020

Update syntax to use 'deriving via' syntax JordanMartinez/purescript-jordans-reference#459

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paf31 reviewed Mar 30, 2020

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kl0tl commented Mar 30, 2020

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kl0tl force-pushed the deriving-via branch from 25050ef to 1eb3791 Compare April 6, 2020 19:05

kl0tl mentioned this pull request Apr 14, 2020

Add a newtype implementing generic instances for use with deriving via purescript-deprecated/purescript-generics-rep#37

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Support deriving via

b8e4ace

kl0tl force-pushed the deriving-via branch from 1eb3791 to b8e4ace Compare April 22, 2020 21:52

JordanMartinez mentioned this pull request Dec 25, 2020

Add UsingGeneric newtype purescript/purescript-prelude#239

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kl0tl mentioned this pull request Dec 26, 2020

[generics-rep] Newtypes for deriving via purescript/purescript-prelude#241

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jrairigh mentioned this pull request Apr 7, 2021

Allow forall in polymorphic instance heads #1120

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rhendric mentioned this pull request Sep 16, 2022

Add compiler-support for deriving Debug type class #4390

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This was referenced Apr 3, 2026

Add deriving clauses to data and newtype declarations i-am-the-slime/purescript#1

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Add derive clauses to data and newtype declarations #4592

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                 'then'             { SourceToken _ (TokLowerName [] "then") }
                 'true'             { SourceToken _ (TokLowerName [] "true") }
                 'type'             { SourceToken _ (TokLowerName [] "type") }
+                'via'              { SourceToken _ (TokLowerName [] "via") }

-                gSimple (MissingNewtypeSuperclassInstance cl1 cl2 ts) = MissingNewtypeSuperclassInstance cl1 cl2 <$> traverse f ts
-                gSimple (UnverifiableSuperclassInstance cl1 cl2 ts) = UnverifiableSuperclassInstance cl1 cl2 <$> traverse f ts
+                gSimple (InvalidViaType cl ts viaTy) = InvalidViaType cl <$> traverse f ts <*> f viaTy
+                gSimple (InvalidViaKind cl ts viaTy viaKind expectedKind) = InvalidViaKind cl <$> traverse f ts <*> f viaTy <*> f viaKind <*> f expectedKind

Support deriving via by kl0tl · Pull Request #3824 · purescript/purescript · GitHub

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+                            , solverDeferErrors      = False
+                            }
+                    void . flip runStateT M.empty . runWriterT $
+                      entails solverOptions constraint M.empty []

Support deriving via by kl0tl · Pull Request #3824 · purescript/purescript · GitHub

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Support deriving via - #3824

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Support deriving via#3824
kl0tl wants to merge 1 commit into
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kl0tl:deriving-via

kl0tl commented Mar 27, 2020 •
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