CallStack naming

[Simon Peyton Jones]

|  foo x :: AppendsCallStack => a -> a

Remove the "x"!

S

|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of
|  Joachim Breitner
|  Sent: 20 January 2016 09:08
|  To: ghc-devs at haskell.org
|  Subject: Re: CallStack naming
|  
|  Hi,
|  
|  Am Mittwoch, den 20.01.2016, 00:39 -0500 schrieb Richard Eisenberg:
|  > I'm sure there's an easy answer to this, but I'm wondering: why is
|  the
|  > CallStack feature implemented with implicit parameters instead of
|  just
|  > a magical constraint? Whenever I use this feature, I don't want to
|  > have to enable -XImplicitParams and then make sure I get the name
|  > right. What would be wrong with, e.g.,
|  >
|  > > undefined :: AppendsCallStack => a
|  >
|  > Seems simpler. Is it problems with a nullary class?
|  
|  I tried to construct this using what we have right now:
|  
|  ==> AppendCallStack.hs <==
|  {-# LANGUAGE ConstraintKinds, ImplicitParams #-} module
|  AppendCallStack (AppendsCallStack) where
|  
|  import GHC.Stack
|  
|  type AppendsCallStack = ?x::CallStack
|  
|  ==> Bar.hs <==
|  module Bar where
|  
|  import AppendCallStack
|  
|  foo x :: AppendsCallStack => a -> a
|  foo x = error "Test"
|  
|  But with GHC-7.10 I get
|  
|      [1 of 2] Compiling AppendCallStack  ( AppendCallStack.hs,
|  AppendCallStack.o )
|      [2 of 2] Compiling Bar              ( Bar.hs, Bar.o )
|      Bar.hs:5:1:
|          Invalid type signature: foo x :: AppendsCallStack => a -> a
|          Should be of form <variable> :: <type>
|  
|  although the constraint seems to be a constraint all right:
|  
|      *AppendCallStack> :kind AppendsCallStack
|      AppendsCallStack :: GHC.Prim.Constraint
|  
|  and with GHC HEAD I get
|  
|      [1 of 2] Compiling AppendCallStack  ( AppendCallStack.hs,
|  AppendCallStack.o )
|  
|      AppendCallStack.hs:6:1: error:
|          • Illegal implicit parameter ‘?x::CallStack’
|          • In the type synonym declaration for ‘AppendsCallStack’
|  
|  Too bad...
|  
|  Greetings,
|  Joachim
|  
|  --
|  Joachim “nomeata” Breitner
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