[Haskell-cafe] netwire example error message

joachifm at fastmail.fm joachifm at fastmail.fm
Sun Mar 22 06:15:09 UTC 2015

On Sun, Mar 22, 2015, at 05:57 AM, briand at aracnet.com wrote:
> The error message:
> time.hs:13:8:
>     No instance for (Show e0) arising from a use of ‘testWire’
>     The type variable ‘e0’ is ambiguous
>     Note: there are several potential instances:
>       instance Show Double -- Defined in ‘GHC.Float’
>       instance Show Float -- Defined in ‘GHC.Float’
>       instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
>         -- Defined in ‘GHC.Real’
>       ...plus 45 others
>     In the expression: testWire clockSession_ pos
>     In an equation for ‘main’: main = testWire clockSession_ pos
> Failed, modules loaded: none.

The problem is that `testWire` wants to print the inhibition value but
has no way of choosing which `Show` instance to use, because the type of
`e` is left unspecified in your snippet. This is obvious if you look at
the constraints on `testWire`, but not so obvious from the error
message. The solution is to fix the inhibition type to `()` (or

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