TH Show instance not working.

[Li-yao Xia]

Hi David,

mkShow is probably making two separate declarations for "show" ([FunD 
"show" _, FunD "show" _]) instead of one declaration with two clauses 
([FunD "show" [Clause ..., Clause ...]]).
Both pretty-print to the same text, but only the second one is actually 
valid. When there is only one constructor, both alternatives end up the 
same.

Li-yao

On 08/20/2017 02:16 AM, David Banas wrote:
> Hi all,
>
> Does anyone know why this code:
>
> module Language.P4.UtilTest where
>
> import Language.P4.Util (mkShow)
>
> data Dummy = Bogus    Char
>             | Nonsense Int
>
> $(mkShow ''Dummy)
>
> is producing this error:
>
> Davids-Air-2:P4 dbanas$ stack ghc -- UtilTest.hs -ddump-splices
> [1 of 1] Compiling Language.P4.UtilTest ( UtilTest.hs, UtilTest.o )
> UtilTest.hs:24:3-16: Splicing declarations
>      mkShow ''Dummy
>    ======>
>      instance Show Dummy where
>        show (Bogus x) = show x
>        show (Nonsense x) = show x
>
> UtilTest.hs:24:3: error:
>      Conflicting definitions for ‘show’
>      Bound at: UtilTest.hs:24:3-16
>                UtilTest.hs:24:3-16
>     |
> 24 | $(mkShow ''Dummy)
>     |   ^^^^^^^^^^^^^^
>
> ?
>
> The TH splice expansion looks correct to me.
> If I comment out the second constructor (Nonsense Int), the code compiles without error.
>
> Thanks,
> -db
>
>
>
>
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