[GHC] #9018: GHC suppresses too much kind information
#9018: GHC suppresses too much kind information ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- GHC tends to hide important kind polymorphism in error messages and in GHCi. (I came across this when looking at #9017.) For example, try this: {{{ bash$ cat Test.hs {-# LANGUAGE PolyKinds #-} module Test where foo :: m a foo = foo bash$ ghci Test.hs ghci> :t foo foo :: m a ghci> :i foo foo :: m a bash$ ghci Test.hs -XPolyKinds ghci> :t foo foo :: m a ghci> :i foo goo :: m a bash$ ghci Test.hs -fprint-explicit-foralls -fprint-explicit-kinds ghci> :t foo foo :: foo :: forall (m :: * -> *) d. m d ghci> :i foo foo :: forall (k :: BOX) (m :: k -> *) (d :: k). m d bash$ ghci Test.hs -XPolyKinds -fprint-explicit-foralls -fprint-explicit- kinds ghci> :t foo foo :: forall (k :: BOX) (m :: k -> *) (d :: k). m d ghci> :i foo foo :: forall (k :: BOX) (m :: k -> *) (d :: k). m d }}} Pretty confusing eh? * Without `-fprint-explicit-foralls -fprint-explicit-kinds` you don't see the kinds on the types at all. * Without `-XPolyKinds` in GHCi, when you say `:t foo` you are asking for the type of the expression `foo` (it could be an arbitrary expression). So `foo` is instantiated and then re-generalised; but without `-XPolyKinds` we don't get a poly-kinded type. Hence the difference in what is printed by `:type` and `:info`. Here's a proposal: even without `-fprint-explicit-foralls`, we should print any foralls that bind a type variable whose kind includes a kind variable. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9018 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9018: GHC suppresses too much kind information -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by goldfire): +1 from me. I lived without a .ghci file until I experienced pain from this particular issue. Now, I have {{{ :seti -XPolyKinds :set -fprint-explicit-foralls :set -fprint-explicit-kinds }}} among other options. Perhaps it might also be nice to inform the user to use `-fprint-explicit- kinds` in an error message if some heuristics (don't know what, at the moment) indicate that it might be helpful. I feel like I've answered several invalid bug reports because of quite legitimately confusing error messages. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9018#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9018: GHC suppresses too much kind information
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Reporter: simonpj | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.8.2
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture: Unknown/Multiple
Type of failure: None/Unknown | Difficulty: Unknown
Test Case: | Blocked By:
Blocking: | Related Tickets:
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Comment (by Simon Peyton Jones
#9018: GHC suppresses too much kind information -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by simonpj): * status: new => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9018#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#9018: GHC suppresses too much kind information -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: feature request | Status: closed Priority: normal | Milestone: 7.8.3 Component: Compiler | Version: 7.8.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by thoughtpolice): * milestone: => 7.8.3 Comment: This was merged into GHC 7.8 for 7.8.3 (part of fixing #9175). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9018#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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