Use Fuzz.oneOfValues when possible

Fixes gampleman#17

src/Internal/Builtin/Fuzzer.elm

@@ -2,6 +2,9 @@ module Internal.Builtin.Fuzzer exposing (codeGen)
 
 import CodeGenerator exposing (CodeGenerator)
 import Elm.CodeGen as CG
+import Elm.Syntax.Expression as Expression
+import Elm.Syntax.Node exposing (Node(..))
+import Maybe.Extra
 import String.Extra
 import TypePattern exposing (TypePattern(..))
 
@@ -19,7 +22,15 @@ codeGen =
         , typePattern = Typed [ "Fuzz" ] "Fuzzer" [ Target ]
         , makeName = \name -> String.Extra.decapitalize name ++ "Fuzzer"
         }
-        [ CodeGenerator.customType (\_ exps -> CG.apply [ fuzz "oneOf", CG.list (List.map Tuple.second exps) ])
+        [ CodeGenerator.customType
+            (\_ exps ->
+                case Maybe.Extra.combineMap extractFromConstant exps of
+                    Just constants ->
+                        CG.apply [ fuzz "oneOfValues", CG.list constants ]
+
+                    Nothing ->
+                        CG.apply [ fuzz "oneOf", CG.list (List.map Tuple.second exps) ]
+            )
         , CodeGenerator.pipeline (\c -> CG.apply [ fuzz "constant", c ]) (\m -> CG.apply [ fuzz "andMap", m ])
         , CodeGenerator.mapN 8 (\name a bs -> CG.apply (fuzz name :: a :: bs))
         , CodeGenerator.map (\a b -> CG.apply [ fuzz "map", a, b ])
@@ -31,3 +42,13 @@ codeGen =
         , CodeGenerator.char (fuzz "char")
         , CodeGenerator.lambdaBreaker (\inner -> CG.apply [ fuzz "lazy", CG.lambda [ CG.unitPattern ] inner ])
         ]
+
+
+extractFromConstant : ( String, CG.Expression ) -> Maybe CG.Expression
+extractFromConstant ( _, expr ) =
+    case expr of
+        Expression.Application [ Node _ (Expression.FunctionOrValue [ "Fuzz" ] "constant"), Node _ inner ] ->
+            Just inner
+
+        _ ->
+            Nothing

tests/FuzzerCodeGenTest.elm

@@ -143,5 +143,27 @@ columnConstraintFuzzer =
 innerColumnConstraintFuzzer : Fuzzer CreateTable.InnerColumnConstraint
 innerColumnConstraintFuzzer =
     Debug.todo ""
+"""
+        , codeGenTest "Issue #17"
+            [ elmJson ]
+            []
+            [ """module A exposing (..)
+import Fuzz exposing (Fuzzer)
+
+type Semaphore = Red | Yellow | Green
+
+fuzzer : Fuzzer Semaphore
+fuzzer =
+    Debug.todo ""
+"""
+            ]
+            """module A exposing (..)
+import Fuzz exposing (Fuzzer)
+
+type Semaphore = Red | Yellow | Green
+
+fuzzer : Fuzzer Semaphore
+fuzzer =
+    Fuzz.oneOfValues [ Red, Yellow, Green ]
 """
         ]