[Haskell-cafe] Strange error message with PolyKinds

[Jonas Scholl]

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I would guess the two arguments are a kind and a type of that kind. The
kind is given implicitly, so this corresponds to your one given
argument. If we specify a kind, we get the following:

Prelude> :set -XPolyKinds -XDataKinds -XKindSignatures
Prelude> type Test a = a
Prelude> type Huh a = Test (a :: Bool)
Prelude> type Bar = Huh

<interactive>:5:1:
    Type synonym ‘Huh’ should have 1 argument, but has been given none
    In the type declaration for ‘Bar’

So it really takes two arguments and we can specify one, leaving one to
be specified. Maybe a better error message would be useful, but I would
not say this is a bug.

On 07/17/2015 08:37 PM, Nikolay Amiantov wrote:
> Hi Cafe, > > I've bumped into a strange error message -- it looks like an
obvious > bug, but maybe there's something that I'm just not aware of: >
> GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help > ghci>
type Test a = a > ghci> type Huh = Test > > <interactive>:3:1: >    
Type synonym ‘Test’ should have 1 argument, but has been given none
>     In the type declaration for ‘Huh’ > > So far, so good: > > ghci>
:set -XPolyKinds > ghci> type Test a = a > ghci> type Huh = Test > >
<interactive>:6:1: >     Type synonym ‘Test’ should have 2 arguments,
but has been given 1 >     In the type declaration for ‘Huh’ > > (Notice
the number of arguments reported in the second case) >

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