add extensions

aima-haskell.cabal

@@ -6,6 +6,7 @@ version:             0.1.0.0
 synopsis:            aima
 description:         aima with haskell for myself to exercise
 homepage:            https://github.com/VonFry/aima-haskell
+bug-reports:         https://github.com/VonFry/aima-haskell/issues
 license:             GPL-3
 license-file:        LICENSE
 author:              Vonfry
@@ -17,8 +18,6 @@ extra-source-files:  ChangeLog.md
                   ,  readme.md
 cabal-version:       >=1.10
 
-Tested-With:         GHC == 8.0.2
-bug-reports:         https://github.com/VonFry/aima-haskell/issues
 Tested-With:         GHC == 8.0.2
 
 source-repository    head
@@ -33,6 +32,7 @@ library
   build-depends:       base >=4.9 && <4.10
   hs-source-dirs:      src
   default-language:    Haskell2010
+  default-extensions:  MultiParamTypeClasses
 
 Test-Suite             test-aima
   type:                exitcode-stdio-1.0
@@ -41,3 +41,4 @@ Test-Suite             test-aima
   hs-source-dirs:      tests
   default-language:    Haskell2010
   ghc-options:         -Wall
+  default-extensions:  MultiParamTypeClasses

src/Data/Aima/Tree.hs

@@ -8,7 +8,7 @@ module Data.Aima.Tree where
 -- @t@ is a tree data. It should have a param to save the node type.
 --
 -- I suggest every instance of this class also instance "Eq" and "Ord"
-class Tree t where
+class (Ord (t a)) => Tree t a where
     {-# MINIMAL parentTree, childrentTree, nodeT, insTree, delTree, orderTree #-}
 
     -- | Get the parent node of a tree.