Type-on-hover for type constructors shows linear arrows even if `-XLinearTypes` is not enabled

[akrmn]

This is probably confusing for newcomers and those who don't know about LinearTypes, even if technically correct according to the user's guide

By default, all fields in algebraic data types are linear
-- https://downloads.haskell.org/ghc/9.0.2/docs/html/users_guide/exts/linear_types.html#data-types

GHCi gets around this because, also according to the user's guide,

[...] Given data T1 a = MkT1 a [...] [w]hen used as a value, MkT1 is given a multiplicity-polymorphic type: MkT1 :: forall {m} a. a %m -> T1 a. [...] When displaying types, unless -XLinearTypes is enabled, multiplicity polymorphic functions are printed as regular functions [...]; therefore constructors appear to have regular function types.
-- https://downloads.haskell.org/ghc/9.0.2/docs/html/users_guide/exts/linear_types.html#data-types

The solution, then, would be to use MkT1-the-value for the type-on-hover tooltip, instead of MkT1-the-constructor.

Your environment

Which OS do you use: macOS
Which LSP client (editor/plugin) do you use: VSCode
Describe your project (alternative: link to the project): plain cabal project, ghc-9.0.2

Steps to reproduce

Define a newtype or data type and hover on one of its constructors.

newtype WrappedInt = MkWrapped Int

Expected behaviour

The tooltip shows the type MkWrapped :: Int -> WrappedInt

Actual behaviour

The tooltip shows the type MkWrapped :: Int %1 -> WrappedInt, which includes a linear arrow.

[michaelpj]

cc @July541, who looked a pretty-printing of stuff recently?

[July541]

A quick peek: we rely on printOutputable to print type info. I'll dive into it later.

haskell-language-server/ghcide/src/Development/IDE/Spans/AtPoint.hs

Lines 250 to 252 in b1bf549

	prettyType t = case kind of
	HieFresh -> printOutputable t
	HieFromDisk full_file -> printOutputable $ hieTypeToIface $ recoverFullType t (hie_types full_file)

[akrmn]

Just wanted to add that I thought I'd check what happens when the constructor is used as a value, and to my surprise that also shows the linear arrows,

mkWrapped = MkWrapped
--          ^^^^^^^^^~ MkWrapped :: Int %1 -> WrappedInt

hovering on mkWrapped does show the expected non-linear arrow, though.

[July541]

It's from

haskell-language-server/ghcide/src/Development/IDE/Spans/AtPoint.hs

Lines 232 to 238 in b1bf549

	prettyName (Right n, dets) = T.unlines $
	wrapHaskell (printOutputable n <> maybe "" (" :: " <>) ((prettyType <$> identType dets) <|> maybeKind))
	: definedAt n
	++ maybeToList (prettyPackageName n)
	++ catMaybes [ T.unlines . spanDocToMarkdown <$> lookupNameEnv dm n
	]
	where maybeKind = fmap printOutputable $ safeTyThingType =<< lookupNameEnv km n

maybeKind if we have newtype WrappedInt = MkWrapped Int and hover on MkWrapped. And it is something about (AConLike (RealDataCon dataCon))

haskell-language-server/ghcide/src/Development/IDE/Spans/Common.hs

Lines 40 to 49 in b1bf549

	safeTyThingType :: TyThing -> Maybe Type
	safeTyThingType thing
	| Just i <- safeTyThingId thing = Just (varType i)
	safeTyThingType (ATyCon tycon) = Just (tyConKind tycon)
	safeTyThingType _ = Nothing
	safeTyThingId :: TyThing -> Maybe Id
	safeTyThingId (AnId i) = Just i
	safeTyThingId (AConLike (RealDataCon dataCon)) = Just (dataConWrapId dataCon)
	safeTyThingId _ = Nothing

We will get linear arrow If we print RealDataCon directly, and it can be disabled by dataConNonlinearType.

I Don't know how to control the printing in showSDoc, A potential solution is reading the dynflag about lineartype to control the behavior by dataConNonlinearType, not sure there exists a better way.