Opened 21 months ago

Last modified 18 months ago

#14877 new bug

QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)'

Reported by: Iceland_jack Owned by:
Priority: normal Milestone:
Component: Compiler Version: 8.5
Keywords: QuantifiedConstraints Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


This works with the latest change to the -XQuantifiedConstraints branch.

{-# Language QuantifiedConstraints, FlexibleInstances, MultiParamTypeClasses, AllowAmbiguousTypes, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-}

class    (forall xx. (xx => a) => Implies xx b) => F a b
instance (forall xx. (xx => a) => Implies xx b) => F a b

class    (a => b) => Implies a b 
instance (a => b) => Implies a b

but replacing Implies xx b with (xx => b) causes it to fail. I don't know if the cause of this overlaps with an existing ticket.

class    (forall xx. (xx => a) => (xx => b)) => F a b
instance (forall xx. (xx => a) => (xx => b)) => F a b

-- $ ghci -ignore-dot-ghci /tmp/ASD.hs 
-- GHCi, version 8.5.20180128:  :? for help
-- [1 of 1] Compiling Main             ( /tmp/ASD.hs, interpreted )
-- /tmp/ASD.hs:4:10: error:
--     • Could not deduce: xx0
--         arising from the superclasses of an instance declaration
--       from the context: forall (xx :: Constraint). (xx => a, xx) => b
--         bound by the instance declaration at /tmp/ASD.hs:4:10-53
--       or from: (xx => a, xx)
--         bound by a quantified context at /tmp/ASD.hs:1:1
--     • In the instance declaration for ‘F a b’
--   |
-- 4 | instance (forall xx. (xx => a) => (xx => b)) => F a b
--   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- Failed, no modules loaded.
-- Prelude> 

Change History (3)

comment:1 Changed 21 months ago by Iceland_jack


comment:2 Changed 21 months ago by simonpj

Consider the quantified constraint (QC) in

instance (forall xx. (xx => a) => Implies xx b)
      => F a b

It serves as a local instance declaration. To construct the dictionary for (F a b) I may need to to solve a constraint (Implies xx b). The QC says "if you want to solve Implies xx b, then it suffices to prove xx => a". And to prove xx => a, the built-in rules assume xx and try to prove a.

It's quite different to say

instance (forall xx. (xx => a) => (xx => b)
      => F a b

That is precisely equivalant to

instance (forall xx. (xx => a, xx) => b)
      => F a b

Now the QC says "if you want to solve the goal b, then conjure up some constraint xx, and prove (xx => a, xx). But how can we guess xx?

Each QC has an instance "head", usually headed by a type constructor, sometimes by a type variable. That head is the pattern the QC can solve. Implies xx b is a good instance head: it matches things of that form. Plain b is not a good instance head!

In short, I see nothing wrong here.

comment:3 Changed 18 months ago by RyanGlScott

Keywords: wipT2893 removed
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