Fixes for GHC 8.4.1 compatibility

* In GHC 8.4.1, the operator <> has become a method of the Semigroup class
  and is exported from the Prelude. This is unfortunate, since <> is also
  exported from the standard library module Text.PrettyPrint, so in any
  module that defines a pretty printer, there is likely to be an ambiguity.

  This affects ~18 modules in GF. Solution:

    import Prelude hiding (<>)

  This works also in older versions of GHC, since GHC does't complain if
  you hide something that doesn't exists.

* In GHC 8.4.1, Semigroup has become a superclass of Monoid. This means
  that anywhere you define an instance of the Monoid class you also have to
  define an instance in the Semigroup class.

  This affects Data.Binary.Builder in GF. Solution: conditionally define
  a Semigroup instance if compiling with base>=4.11 (ghc>=8.4.1)

3 files changed, 3 insertions, 0 deletions

diff --git a/src/compiler/GF/Compile/CheckGrammar.hs b/src/compiler/GF/Compile/CheckGrammar.hs
index 5c1743b74..1348d8e41 100644
--- a/src/compiler/GF/Compile/CheckGrammar.hs
+++ b/src/compiler/GF/Compile/CheckGrammar.hs
@@ -21,6 +21,7 @@
 -----------------------------------------------------------------------------
 
 module GF.Compile.CheckGrammar(checkModule) where
+import Prelude hiding ((<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 
 import GF.Infra.Ident
 import GF.Infra.Option
diff --git a/src/compiler/GF/Compile/Compute/ConcreteNew.hs b/src/compiler/GF/Compile/Compute/ConcreteNew.hs
index a77da88bf..f9edc931c 100644
--- a/src/compiler/GF/Compile/Compute/ConcreteNew.hs
+++ b/src/compiler/GF/Compile/Compute/ConcreteNew.hs
@@ -5,6 +5,7 @@ module GF.Compile.Compute.ConcreteNew
             normalForm,
             Value(..), Bind(..), Env, value2term, eval, vapply
            ) where
+import Prelude hiding ((<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 
 import GF.Grammar hiding (Env, VGen, VApp, VRecType)
 import GF.Grammar.Lookup(lookupResDefLoc,allParamValues)
diff --git a/src/compiler/GF/Compile/TypeCheck/RConcrete.hs b/src/compiler/GF/Compile/TypeCheck/RConcrete.hs
index 2fe08b256..88e324ff3 100644
--- a/src/compiler/GF/Compile/TypeCheck/RConcrete.hs
+++ b/src/compiler/GF/Compile/TypeCheck/RConcrete.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE PatternGuards #-}
 module GF.Compile.TypeCheck.RConcrete( checkLType, inferLType, computeLType, ppType ) where
+import Prelude hiding ((<>)) -- GHC 8.4.1 clash with Text.PrettyPrint
 
 import GF.Infra.CheckM
 import GF.Data.Operations