[GHC] #11459: Rather terrible error message due to excessive kind polymorphism
#11459: Rather terrible error message due to excessive kind polymorphism -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When fixing up `cassava` for GHC 8.0 I found I needed to enable `PolyKinds` due to an unrelated change encountered a rather vexing error. Consider this, {{{#!hs {-# LANGUAGE DataKinds, PolyKinds, KindSignatures, RankNTypes #-} module Hi where -- | Failure continuation. type Failure f r = String -> f r -- | Success continuation. type Success a f r = a -> f r newtype Parser a = Parser { unParser :: forall f r. Failure f r -> Success a f r -> f r } runParser :: Parser a -> Either String a runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}} With GHC 7.10 this failed with the quite comprehensible, {{{ Hi.hs:21:20: A newtype constructor cannot have existential type variables Parser :: forall a (k :: BOX). (forall (f :: k -> *) (r :: k). Failure f r -> Success a f r -> f r) -> Parser a In the definition of data constructor ‘Parser’ In the newtype declaration for ‘Parser’ }}} However, with 8.0 the compiler curtly informs you that, {{{ Hi.hs:29:26: error: • Couldn't match kind ‘GHC.Prim.Any’ with ‘*’ When matching the kind of ‘Either String’ • In the second argument of ‘unParser’, namely ‘left’ In the expression: unParser p left right In an equation for ‘runParser’: runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}} As expected, adding a kind signature to `Parser`'s type variables fixed the issue but the error doesn't help the user realize this nearly as much as it could. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11459 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11459: Rather terrible error message due to excessive kind polymorphism -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: Old description:
When fixing up `cassava` for GHC 8.0 I found I needed to enable `PolyKinds` due to an unrelated change encountered a rather vexing error.
Consider this, {{{#!hs {-# LANGUAGE DataKinds, PolyKinds, KindSignatures, RankNTypes #-}
module Hi where
-- | Failure continuation. type Failure f r = String -> f r -- | Success continuation. type Success a f r = a -> f r
newtype Parser a = Parser { unParser :: forall f r. Failure f r -> Success a f r -> f r }
runParser :: Parser a -> Either String a runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}}
With GHC 7.10 this failed with the quite comprehensible, {{{ Hi.hs:21:20: A newtype constructor cannot have existential type variables Parser :: forall a (k :: BOX). (forall (f :: k -> *) (r :: k). Failure f r -> Success a f r -> f r) -> Parser a In the definition of data constructor ‘Parser’ In the newtype declaration for ‘Parser’ }}}
However, with 8.0 the compiler curtly informs you that, {{{ Hi.hs:29:26: error: • Couldn't match kind ‘GHC.Prim.Any’ with ‘*’ When matching the kind of ‘Either String’ • In the second argument of ‘unParser’, namely ‘left’ In the expression: unParser p left right In an equation for ‘runParser’: runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}}
As expected, adding a kind signature to `Parser`'s type variables fixed the issue but the error doesn't help the user realize this nearly as much as it could.
New description: When fixing up `cassava` for GHC 8.0 I found I needed to enable `PolyKinds` due to an unrelated change (namely in order to apply `Proxy` to something of kind `GHC.Generics.Meta`, which will be quite a common refactoring in 8.0) encountered a rather vexing error. Consider this, {{{#!hs {-# LANGUAGE DataKinds, PolyKinds, KindSignatures, RankNTypes #-} module Hi where -- | Failure continuation. type Failure f r = String -> f r -- | Success continuation. type Success a f r = a -> f r newtype Parser a = Parser { unParser :: forall f r. Failure f r -> Success a f r -> f r } runParser :: Parser a -> Either String a runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}} With GHC 7.10 this failed with the quite comprehensible, {{{ Hi.hs:21:20: A newtype constructor cannot have existential type variables Parser :: forall a (k :: BOX). (forall (f :: k -> *) (r :: k). Failure f r -> Success a f r -> f r) -> Parser a In the definition of data constructor ‘Parser’ In the newtype declaration for ‘Parser’ }}} However, with 8.0 the compiler curtly informs you that, {{{ Hi.hs:29:26: error: • Couldn't match kind ‘GHC.Prim.Any’ with ‘*’ When matching the kind of ‘Either String’ • In the second argument of ‘unParser’, namely ‘left’ In the expression: unParser p left right In an equation for ‘runParser’: runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}} As expected, adding a kind signature to `Parser`'s type variables fixed the issue but the error doesn't help the user realize this nearly as much as it could. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11459#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11459: Rather terrible error message due to excessive kind polymorphism -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: Old description:
When fixing up `cassava` for GHC 8.0 I found I needed to enable `PolyKinds` due to an unrelated change (namely in order to apply `Proxy` to something of kind `GHC.Generics.Meta`, which will be quite a common refactoring in 8.0) encountered a rather vexing error.
Consider this, {{{#!hs {-# LANGUAGE DataKinds, PolyKinds, KindSignatures, RankNTypes #-}
module Hi where
-- | Failure continuation. type Failure f r = String -> f r -- | Success continuation. type Success a f r = a -> f r
newtype Parser a = Parser { unParser :: forall f r. Failure f r -> Success a f r -> f r }
runParser :: Parser a -> Either String a runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}}
With GHC 7.10 this failed with the quite comprehensible, {{{ Hi.hs:21:20: A newtype constructor cannot have existential type variables Parser :: forall a (k :: BOX). (forall (f :: k -> *) (r :: k). Failure f r -> Success a f r -> f r) -> Parser a In the definition of data constructor ‘Parser’ In the newtype declaration for ‘Parser’ }}}
However, with 8.0 the compiler curtly informs you that, {{{ Hi.hs:29:26: error: • Couldn't match kind ‘GHC.Prim.Any’ with ‘*’ When matching the kind of ‘Either String’ • In the second argument of ‘unParser’, namely ‘left’ In the expression: unParser p left right In an equation for ‘runParser’: runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}}
As expected, adding a kind signature to `Parser`'s type variables fixed the issue but the error doesn't help the user realize this nearly as much as it could.
New description: When fixing up `cassava` for GHC 8.0 I found I needed to enable `PolyKinds` due to an unrelated change (namely in order to apply `Proxy` to something of kind `GHC.Generics.Meta`, which will be quite a common refactoring in 8.0) encountered a rather vexing error. Consider this, {{{#!hs {-# LANGUAGE DataKinds, PolyKinds, KindSignatures, RankNTypes #-} module Hi where -- | Failure continuation. type Failure f r = String -> f r -- | Success continuation. type Success a f r = a -> f r newtype Parser a = Parser { unParser :: forall f r. Failure f r -> Success a f r -> f r } runParser :: Parser a -> Either String a runParser p = unParser p Left Right }}} With GHC 7.10 this failed with the quite comprehensible, {{{ Hi.hs:21:20: A newtype constructor cannot have existential type variables Parser :: forall a (k :: BOX). (forall (f :: k -> *) (r :: k). Failure f r -> Success a f r -> f r) -> Parser a In the definition of data constructor ‘Parser’ In the newtype declaration for ‘Parser’ }}} However, with 8.0 the compiler curtly informs you that, {{{ Hi.hs:29:26: error: • Couldn't match kind ‘GHC.Prim.Any’ with ‘*’ When matching the kind of ‘Either String’ • In the second argument of ‘unParser’, namely ‘left’ In the expression: unParser p left right In an equation for ‘runParser’: runParser p = unParser p left right where left !errMsg = Left errMsg right !x = Right x }}} As expected, adding a kind signature to `Parser`'s type variables fixed the issue but the error doesn't help the user realize this nearly as much as it could. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11459#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11459: Rather terrible error message due to excessive kind polymorphism
-------------------------------------+-------------------------------------
Reporter: bgamari | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 8.0.1
Component: Compiler | Version: 7.10.3
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Incorrect | Unknown/Multiple
warning at compile-time | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
-------------------------------------+-------------------------------------
Comment (by Simon Peyton Jones
#11459: Rather terrible error message due to excessive kind polymorphism -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: merge Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect | Test Case: warning at compile-time | polykinds/T11459 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * testcase: => polykinds/T11459 * status: new => merge Comment: Thanks for reporting this! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11459#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11459: Rather terrible error message due to excessive kind polymorphism -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.3 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect | Test Case: warning at compile-time | polykinds/T11459 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: merge => closed * resolution: => fixed Comment: Merged to `ghc-8.0` as e7e2ac8b740264527dd530c35cbbb6b63dc93640. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11459#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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