Linear types (#15981)

This is the first step towards implementation of the linear types proposal
(https://github.com/ghc-proposals/ghc-proposals/pull/111).

It features

* A language extension -XLinearTypes
* Syntax for linear functions in the surface language
* Linearity checking in Core Lint, enabled with -dlinear-core-lint
* Core-to-core passes are mostly compatible with linearity
* Fields in a data type can be linear or unrestricted; linear fields
  have multiplicity-polymorphic constructors.
  If -XLinearTypes is disabled, the GADT syntax defaults to linear fields

The following items are not yet supported:

* a # m -> b syntax (only prefix FUN is supported for now)
* Full multiplicity inference (multiplicities are really only checked)
* Decent linearity error messages
* Linear let, where, and case expressions in the surface language
  (each of these currently introduce the unrestricted variant)
* Multiplicity-parametric fields
* Syntax for annotating lambda-bound or let-bound with a multiplicity
* Syntax for non-linear/multiple-field-multiplicity records
* Linear projections for records with a single linear field
* Linear pattern synonyms
* Multiplicity coercions (test LinearPolyType)

A high-level description can be found at
https://ghc.haskell.org/trac/ghc/wiki/LinearTypes/Implementation
Following the link above you will find a description of the changes made to Core.
This commit has been authored by

* Richard Eisenberg
* Krzysztof Gogolewski
* Matthew Pickering
* Arnaud Spiwack

With contributions from:

* Mark Barbone
* Alexander Vershilov

Updates haddock submodule.

1 files changed, 9 insertions, 3 deletions

diff --git a/compiler/GHC/Utils/Outputable.hs b/compiler/GHC/Utils/Outputable.hs
index 0514870cea..809c06b64d 100644
--- a/compiler/GHC/Utils/Outputable.hs
+++ b/compiler/GHC/Utils/Outputable.hs
@@ -28,8 +28,8 @@ module GHC.Utils.Outputable (
         parens, cparen, brackets, braces, quotes, quote,
         doubleQuotes, angleBrackets,
         semi, comma, colon, dcolon, space, equals, dot, vbar,
-        arrow, larrow, darrow, arrowt, larrowt, arrowtt, larrowtt,
-        lparen, rparen, lbrack, rbrack, lbrace, rbrace, underscore,
+        arrow, lollipop, larrow, darrow, arrowt, larrowt, arrowtt, larrowtt,
+        lparen, rparen, lbrack, rbrack, lbrace, rbrace, underscore, mulArrow,
         blankLine, forAllLit, bullet,
         (<>), (<+>), hcat, hsep,
         ($$), ($+$), vcat,
@@ -357,6 +357,7 @@ data SDocContext = SDC
   , sdocSuppressStgExts             :: !Bool
   , sdocErrorSpans                  :: !Bool
   , sdocStarIsType                  :: !Bool
+  , sdocLinearTypes                 :: !Bool
   , sdocImpredicativeTypes          :: !Bool
   , sdocDynFlags                    :: DynFlags -- TODO: remove
   }
@@ -644,12 +645,13 @@ quotes d = sdocOption sdocCanUseUnicode $ \case
         _other              -> Pretty.quotes pp_d
 
 semi, comma, colon, equals, space, dcolon, underscore, dot, vbar :: SDoc
-arrow, larrow, darrow, arrowt, larrowt, arrowtt, larrowtt :: SDoc
+arrow, lollipop, larrow, darrow, arrowt, larrowt, arrowtt, larrowtt :: SDoc
 lparen, rparen, lbrack, rbrack, lbrace, rbrace, blankLine :: SDoc
 
 blankLine  = docToSDoc $ Pretty.text ""
 dcolon     = unicodeSyntax (char '∷') (docToSDoc $ Pretty.text "::")
 arrow      = unicodeSyntax (char '→') (docToSDoc $ Pretty.text "->")
+lollipop   = unicodeSyntax (char '⊸') (docToSDoc $ Pretty.text "#->")
 larrow     = unicodeSyntax (char '←') (docToSDoc $ Pretty.text "<-")
 darrow     = unicodeSyntax (char '⇒') (docToSDoc $ Pretty.text "=>")
 arrowt     = unicodeSyntax (char '⤚') (docToSDoc $ Pretty.text ">-")
@@ -671,6 +673,10 @@ rbrack     = docToSDoc $ Pretty.rbrack
 lbrace     = docToSDoc $ Pretty.lbrace
 rbrace     = docToSDoc $ Pretty.rbrace
 
+mulArrow :: SDoc -> SDoc
+mulArrow d = text "#" <+> d <+> arrow
+
+
 forAllLit :: SDoc
 forAllLit = unicodeSyntax (char '∀') (text "forall")