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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