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# Quantified constraints
This wiki page summarises the state of play on the idea of allowing quantification in class contexts. For example
This wiki page summarises the state of play on the idea of allowing quantification in class constraints. For example
```wiki
data Rose f a = Branch a (f (Rose f a))
......@@ -29,12 +29,140 @@ Here are some resources
## Status
Use Keyword = `QuantifiedContexts` to ensure that a ticket ends up on these lists.
Use Keyword = `QuantifiedConstraints` to ensure that a ticket ends up on these lists.
Open Tickets:
No results
<table><tr><th>[\#8516](https://gitlab.haskell.org//ghc/ghc/issues/8516)</th>
<td>Add (-\>) representation and the Invariant class to GHC.Generics</td></tr>
<tr><th>[\#13153](https://gitlab.haskell.org//ghc/ghc/issues/13153)</th>
<td>Several Traversable instances have an extra fmap</td></tr>
<tr><th>[\#14317](https://gitlab.haskell.org//ghc/ghc/issues/14317)</th>
<td>Solve Coercible constraints over type constructors</td></tr>
<tr><th>[\#14831](https://gitlab.haskell.org//ghc/ghc/issues/14831)</th>
<td>QuantifiedConstraints: Odd superclass constraint</td></tr>
<tr><th>[\#14832](https://gitlab.haskell.org//ghc/ghc/issues/14832)</th>
<td>QuantifiedConstraints: Adding to the context causes failure</td></tr>
<tr><th>[\#14860](https://gitlab.haskell.org//ghc/ghc/issues/14860)</th>
<td>QuantifiedConstraints: Can't quantify constraint involving type family</td></tr>
<tr><th>[\#14877](https://gitlab.haskell.org//ghc/ghc/issues/14877)</th>
<td>QuantifiedConstraints: Can't deduce \`xx' from \`(xx =\> a, xx)'</td></tr>
<tr><th>[\#14879](https://gitlab.haskell.org//ghc/ghc/issues/14879)</th>
<td>QuantifiedConstraints: Big error message + can't substitute (=\>) with a class alias</td></tr>
<tr><th>[\#14896](https://gitlab.haskell.org//ghc/ghc/issues/14896)</th>
<td>QuantifiedConstraints: GHC does doesn't discharge constraints if they are quantified</td></tr>
<tr><th>[\#14937](https://gitlab.haskell.org//ghc/ghc/issues/14937)</th>
<td>QuantifiedConstraints: Reify implication constraints from terms lacking them</td></tr>
<tr><th>[\#14943](https://gitlab.haskell.org//ghc/ghc/issues/14943)</th>
<td>Make (=\>) polykinded (:: k -\> k -\> Constraint)</td></tr>
<tr><th>[\#14958](https://gitlab.haskell.org//ghc/ghc/issues/14958)</th>
<td>QuantifiedConstraints: Doesn't apply implication for existential?</td></tr>
<tr><th>[\#14968](https://gitlab.haskell.org//ghc/ghc/issues/14968)</th>
<td>QuantifiedConstraints: Can't be RHS of type family instances</td></tr>
<tr><th>[\#14983](https://gitlab.haskell.org//ghc/ghc/issues/14983)</th>
<td>Have custom type errors imply Void</td></tr>
<tr><th>[\#14995](https://gitlab.haskell.org//ghc/ghc/issues/14995)</th>
<td>QuantifiedConstraints: Incorrect pretty printing</td></tr>
<tr><th>[\#15347](https://gitlab.haskell.org//ghc/ghc/issues/15347)</th>
<td>QuantifiedConstraints: Implication constraints with type families don't work</td></tr>
<tr><th>[\#15351](https://gitlab.haskell.org//ghc/ghc/issues/15351)</th>
<td>QuantifiedConstraints ignore FunctionalDependencies</td></tr>
<tr><th>[\#15409](https://gitlab.haskell.org//ghc/ghc/issues/15409)</th>
<td>Quantified constraints half-work with equality constraints</td></tr>
<tr><th>[\#15639](https://gitlab.haskell.org//ghc/ghc/issues/15639)</th>
<td>Surprising failure combining QuantifiedConstraints with Coercible</td></tr>
<tr><th>[\#15888](https://gitlab.haskell.org//ghc/ghc/issues/15888)</th>
<td>Quantified constraints can be loopy</td></tr>
<tr><th>[\#16139](https://gitlab.haskell.org//ghc/ghc/issues/16139)</th>
<td>GHC confused about type synonym kind with QuantifiedConstraints</td></tr>
<tr><th>[\#16140](https://gitlab.haskell.org//ghc/ghc/issues/16140)</th>
<td>Cannot create type synonym for quantified constraint without ImpredicativeTypes</td></tr>
<tr><th>[\#16173](https://gitlab.haskell.org//ghc/ghc/issues/16173)</th>
<td>Move \`Data.Profunctor\` from \`profunctors\` package to \`base\`</td></tr>
<tr><th>[\#16245](https://gitlab.haskell.org//ghc/ghc/issues/16245)</th>
<td>GHC panic (No skolem info) with QuantifiedConstraints and strange scoping</td></tr>
<tr><th>[\#16365](https://gitlab.haskell.org//ghc/ghc/issues/16365)</th>
<td>Inconsistency in quantified constraint solving</td></tr></table>
Closed Tickets:
No results
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<table><tr><th>[\#2256](https://gitlab.haskell.org//ghc/ghc/issues/2256)</th>
<td>Incompleteness of type inference: must quantify over implication constraints</td></tr>
<tr><th>[\#2893](https://gitlab.haskell.org//ghc/ghc/issues/2893)</th>
<td>Implement "Quantified constraints" proposal</td></tr>
<tr><th>[\#5927](https://gitlab.haskell.org//ghc/ghc/issues/5927)</th>
<td>A type-level "implies" constraint on Constraints</td></tr>
<tr><th>[\#9123](https://gitlab.haskell.org//ghc/ghc/issues/9123)</th>
<td>Emit quantified Coercible constraints in GeneralizedNewtypeDeriving</td></tr>
<tr><th>[\#12245](https://gitlab.haskell.org//ghc/ghc/issues/12245)</th>
<td>Deriving Data at higher kinds</td></tr>
<tr><th>[\#14070](https://gitlab.haskell.org//ghc/ghc/issues/14070)</th>
<td>Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’</td></tr>
<tr><th>[\#14733](https://gitlab.haskell.org//ghc/ghc/issues/14733)</th>
<td>Won't use (forall xx. f xx) with -XQuantifiedConstraints</td></tr>
<tr><th>[\#14734](https://gitlab.haskell.org//ghc/ghc/issues/14734)</th>
<td>QuantifiedConstraints conflated with impredicative polymorphism?</td></tr>
<tr><th>[\#14735](https://gitlab.haskell.org//ghc/ghc/issues/14735)</th>
<td>GHC Panic with QuantifiedConstraints</td></tr>
<tr><th>[\#14744](https://gitlab.haskell.org//ghc/ghc/issues/14744)</th>
<td>Non-exhaustive patterns in case in GHCi with quantified class contexts</td></tr>
<tr><th>[\#14748](https://gitlab.haskell.org//ghc/ghc/issues/14748)</th>
<td>Infer context for Data instance of (data Foo f = Foo (f Bool) (f Int))</td></tr>
<tr><th>[\#14799](https://gitlab.haskell.org//ghc/ghc/issues/14799)</th>
<td>QuantifiedConstraints: Problems with Typeable</td></tr>
<tr><th>[\#14822](https://gitlab.haskell.org//ghc/ghc/issues/14822)</th>
<td>-XQuantifiedConstraints: Turn term-level entailments (:-) into constraints (=\>)</td></tr>
<tr><th>[\#14833](https://gitlab.haskell.org//ghc/ghc/issues/14833)</th>
<td>QuantifiedConstraints: GHC can't deduce (() :: Constraint)?</td></tr>
<tr><th>[\#14835](https://gitlab.haskell.org//ghc/ghc/issues/14835)</th>
<td>QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b"</td></tr>
<tr><th>[\#14840](https://gitlab.haskell.org//ghc/ghc/issues/14840)</th>
<td>QuantifiedConstraints: Can't define class alias</td></tr>
<tr><th>[\#14861](https://gitlab.haskell.org//ghc/ghc/issues/14861)</th>
<td>QuantifiedConstraints: Can't use forall'd variable in context</td></tr>
<tr><th>[\#14863](https://gitlab.haskell.org//ghc/ghc/issues/14863)</th>
<td>QuantifiedConstraints: Can't deduce \`c' from \`(a, b)' and \`a \|- b \|- c'</td></tr>
<tr><th>[\#14878](https://gitlab.haskell.org//ghc/ghc/issues/14878)</th>
<td>Can't witness transitivity ((.)) of isomorphism of Constraints</td></tr>
<tr><th>[\#14883](https://gitlab.haskell.org//ghc/ghc/issues/14883)</th>
<td>QuantifiedConstraints don't kick in when used in TypeApplications</td></tr>
<tr><th>[\#14897](https://gitlab.haskell.org//ghc/ghc/issues/14897)</th>
<td>QuantifiedConstraints: Can't print type of quantified constraint</td></tr>
<tr><th>[\#14942](https://gitlab.haskell.org//ghc/ghc/issues/14942)</th>
<td>QuantifiedConstraints: GHC can't infer</td></tr>
<tr><th>[\#14961](https://gitlab.haskell.org//ghc/ghc/issues/14961)</th>
<td>QuantifiedConstraints: introducing classes through equality constraints fails</td></tr>
<tr><th>[\#14993](https://gitlab.haskell.org//ghc/ghc/issues/14993)</th>
<td>QuantifiedConstraints and principal types</td></tr>
<tr><th>[\#15008](https://gitlab.haskell.org//ghc/ghc/issues/15008)</th>
<td>Type synonyms with hidden, determined type variables</td></tr>
<tr><th>[\#15231](https://gitlab.haskell.org//ghc/ghc/issues/15231)</th>
<td>UndecidableInstances validity checking is wrong in the presence of QuantifiedConstraints</td></tr>
<tr><th>[\#15244](https://gitlab.haskell.org//ghc/ghc/issues/15244)</th>
<td>Ambiguity checks in QuantifiedConstraints</td></tr>
<tr><th>[\#15290](https://gitlab.haskell.org//ghc/ghc/issues/15290)</th>
<td>QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"</td></tr>
<tr><th>[\#15316](https://gitlab.haskell.org//ghc/ghc/issues/15316)</th>
<td>Regarding coherence and implication loops in presence of QuantifiedConstraints</td></tr>
<tr><th>[\#15334](https://gitlab.haskell.org//ghc/ghc/issues/15334)</th>
<td>(forall x. c x, forall x. d x) is not equivalent to forall x. (c x, d x)</td></tr>
<tr><th>[\#15359](https://gitlab.haskell.org//ghc/ghc/issues/15359)</th>
<td>Quantified constraints do not work with equality constraints</td></tr>
<tr><th>[\#15507](https://gitlab.haskell.org//ghc/ghc/issues/15507)</th>
<td>Deriving with QuantifiedConstraints is unable to penetrate type families</td></tr>
<tr><th>[\#15593](https://gitlab.haskell.org//ghc/ghc/issues/15593)</th>
<td>QuantifiedConstraints: trouble with type family</td></tr>
<tr><th>[\#15625](https://gitlab.haskell.org//ghc/ghc/issues/15625)</th>
<td>GHC panic, with QuantifiedConstraints</td></tr>
<tr><th>[\#15635](https://gitlab.haskell.org//ghc/ghc/issues/15635)</th>
<td>Implication introduction for quantified constraints</td></tr>
<tr><th>[\#15636](https://gitlab.haskell.org//ghc/ghc/issues/15636)</th>
<td>Implication constraint priority breaks default class implementations</td></tr>
<tr><th>[\#15943](https://gitlab.haskell.org//ghc/ghc/issues/15943)</th>
<td>"ASSERT failed" with quantified constraints</td></tr>
<tr><th>[\#15974](https://gitlab.haskell.org//ghc/ghc/issues/15974)</th>
<td>QuantifiedConstraints: Spurious error involving superclass constraints</td></tr>
<tr><th>[\#16123](https://gitlab.haskell.org//ghc/ghc/issues/16123)</th>
<td>QuantifiedConstraints fails to deduce trivial constraint</td></tr></table>
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