add "deriving via" support

Author: raichoo

syntax/haskell.vim

@@ -36,7 +36,12 @@ syn match haskellTypeSig
   \ haskellParens
 syn keyword haskellWhere where
 syn keyword haskellLet let
-syn match HaskellDerive "\<deriving\>\(\s\+\<\(anyclass\|instance\|newtype\|stock\)\>\)\?"
+syn keyword haskellDeriveKeyword deriving anyclass instance newtype stock via contained
+syn match haskellDerive "deriving\(\s\+instance\)\?\(\s\+anyclass\|\s\+newtype\|\s\+stock\|\s\+.\{-}\_s\+via\)\?"
+  \ contains=
+  \ haskellDeriveKeyword,
+  \ haskellParens,
+  \ haskellType
 syn keyword haskellDeclKeyword module class instance newtype in
 syn match haskellDecl "\<\(type\|data\)\>\s\+\(\<family\>\)\?"
 syn keyword haskellDefault default
@@ -161,13 +166,13 @@ highlight def link haskellType Type
 highlight def link haskellImportKeywords Include
 if get(g:, 'haskell_classic_highlighting', 0)
   highlight def link haskellDeclKeyword Keyword
-  highlight def link HaskellDerive Keyword
+  highlight def link haskellDeriveKeyword Keyword
   highlight def link haskellDecl Keyword
   highlight def link haskellWhere Keyword
   highlight def link haskellLet Keyword
 else
   highlight def link haskellDeclKeyword Structure
-  highlight def link HaskellDerive Structure
+  highlight def link haskellDeriveKeyword Structure
   highlight def link haskellDecl Structure
   highlight def link haskellWhere Structure
   highlight def link haskellLet Structure