[GHC] #14303: HasField ambiguity error
#14303: HasField ambiguity error -------------------------------------+------------------------------------- Reporter: cloudhead | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Both of these functions fail to compile with the error below: {{{#!haskell {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} import Control.Monad.Reader import GHC.Records import Data.Proxy askField :: forall x a m r. (HasField x r a, MonadReader r m) => m a askField = asks (getField @x) askField' :: forall x a m r. (HasField x r a, MonadReader r m) => m a askField' = asks (getFieldWithProxy (Proxy :: Proxy x)) where getFieldWithProxy :: forall proxy. proxy x -> r -> a getFieldWithProxy = const getField }}} {{{ Test.hs:12:14: error: • Could not deduce (HasField x0 r a) from the context: (HasField x r a, MonadReader r m) bound by the type signature for: askField' :: forall x a (m :: * -> *) r. (HasField x r a, MonadReader r m) => m a at Test.hs:12:14-69 The type variable ‘x0’ is ambiguous • In the ambiguity check for ‘askField'’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature: askField' :: forall x a m r. (HasField x r a, MonadReader r m) => m a | 12 | askField' :: forall x a m r. (HasField x r a, MonadReader r m) => m a | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} I hope I'm not missing something, but one of these should compile. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14303 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14303: HasField ambiguity error -------------------------------------+------------------------------------- Reporter: cloudhead | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => invalid Comment: This is expected behavior. GHC reports that these constraints are ambiguous because it can't determine what `x` is from the types on the right-hand sides of each function. Notice that `m a` doesn't mention `x`, and `x` isn't determined by a functional dependency like the `r` in `MonadReader r m`. There is a way to make ambiguous type signatures like this compile, however, by using the aptly named extension `AllowAmbiguousTypes` (which is often needed in `TypeApplications`-heavy code like what you have here). With `AllowAmbiguousTypes`, `askField` compiles without any further changes: {{{#!hs {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} import Control.Monad.Reader import GHC.Records import Data.Proxy askField :: forall x a m r. (HasField x r a, MonadReader r m) => m a askField = asks (getField @x) }}} Making `askField'` compile takes a little extra work, since GHC is unable to figure out that you meant to use `getField` at type `x`. To fix this, use another type application: {{{#!hs {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} import Control.Monad.Reader import GHC.Records import Data.Proxy askField' :: forall x a m r. (HasField x r a, MonadReader r m) => m a askField' = asks (getFieldWithProxy (Proxy :: Proxy x)) where getFieldWithProxy :: forall proxy. proxy x -> r -> a getFieldWithProxy = const (getField @x) }}} Which makes it compile as well. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14303#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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