[GHC] #12471: Weirdness when using quosiquoter in pattern synonyms / GADT context

[GHC]

#12471: Weirdness when using quosiquoter in pattern synonyms / GADT context
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  TemplateHaskell, PatternSynonyms,
                                     |  GADTs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Iceland_jack:

@@ -1,1 +1,1 @@
- ~~Using [https://hackage.haskell.org/package/applicative-
+ Using [https://hackage.haskell.org/package/applicative-
@@ -8,8 +8,2 @@
- data Exp a where
-   I :: Int -> Exp Int
-
- pattern MkI :: Int ~ a => a -> Exp a
- pattern MkI a = I a
-
- c :: Exp a -> IO a
- c (MkI n) = pure n
+ c :: Int -> IO Integer
+ c n = [i| fromIntegral n::Integer |]
@@ -18,1 +12,1 @@
- This works. If I replace `pure n` by `[i| n |]` I get this error:
+ This works. Removing the type annotation:
@@ -20,0 +14,6 @@
+ {{{#!hs
+ import Control.Applicative.QQ.Idiom
+
+ c :: Int -> IO Integer
+ c n = [i| fromIntegral n |]
+ }}}
@@ -21,10 +21,5 @@
- tKTa.hs:10:4-8: error: …
-     • Couldn't match type ‘a’ with ‘Int’ arising from a pattern
-       ‘a’ is a rigid type variable bound by
-         the type signature for:
-           c :: forall a. Exp a -> IO a
-         at /tmp/tKTa.hs:9:6
-     • In the pattern: MkI n
-       In an equation for ‘c’: c (MkI n) = (pure n)
-     • Relevant bindings include
-         c :: Exp a -> IO a (bound at /tmp/tKTa.hs:10:1)
+ t0sj.hs:4:10-27: error: …
+     • Couldn't match expected type ‘IO Integer’ with actual type ‘Int’
+     • In the second argument of ‘(<*>)’, namely ‘n’
+       In the expression: (((pure fromIntegral) <*> n))
+       In an equation for ‘c’: c n = (((pure fromIntegral) <*> n))
@@ -33,8 +28,0 @@
-
- This works without the pattern synonym
-
- {{{#!hs
- c :: Exp a -> IO a
- c (I n) = [i| n |]
- }}}
- ~~

New description:

 Using [https://hackage.haskell.org/package/applicative-
 quoters-0.1.0.8/docs/Control-Applicative-QQ-Idiom.html idiom brackets], is
 this a bug?

 {{{#!hs
 import Control.Applicative.QQ.Idiom

 c :: Int -> IO Integer
 c n = [i| fromIntegral n::Integer |]
 }}}

 This works. Removing the type annotation:

 {{{#!hs
 import Control.Applicative.QQ.Idiom

 c :: Int -> IO Integer
 c n = [i| fromIntegral n |]
 }}}
 {{{
 t0sj.hs:4:10-27: error: …
     • Couldn't match expected type ‘IO Integer’ with actual type ‘Int’
     • In the second argument of ‘(<*>)’, namely ‘n’
       In the expression: (((pure fromIntegral) <*> n))
       In an equation for ‘c’: c n = (((pure fromIntegral) <*> n))
 Compilation failed.
 }}}

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12471#comment:3>
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