Don't use data-default-class to build protos.

Fixes google#194.  `data-default` is problematic since it's (1) lawless and (2)
creeps into places it's not really intended for; e.g., using it for an integer
value instead of an explicit 0.  But protobufs *do* have a useful law, namely,
`encodeMessage def == ""`.

This change makes `Data.ProtoLens.Message` (and thus `Data.ProtoLens`, which reexports it):

- Add a `defaultMessage` method to `Message`.
- Redefine `def = defaultMessage` for convenience.

I'm not convinced this is the best long-term UX; suggestions welcome.

proto-lens-combinators/package.yaml

@@ -27,7 +27,7 @@ library:
   exposed-modules:
     - Data.ProtoLens.Combinators
   dependencies:
-    - data-default-class >= 0.0 && < 0.2
+    - proto-lens == 0.4.*
     - transformers >= 0.4 && < 0.6
 
 tests:

proto-lens-combinators/src/Data/ProtoLens/Combinators.hs

@@ -16,7 +16,7 @@ module Data.ProtoLens.Combinators
     ) where
 
 import Control.Monad.Trans.State (State, execState)
-import Data.Default.Class (Default(def))
+import Data.ProtoLens (Message, def)
 import Data.Maybe (isJust)
 import Lens.Family2 (LensLike, Phantom, Setter, to, (.~))
 
@@ -33,7 +33,7 @@ clear = (.~ Nothing)
 
 -- | Creates a 'Default' and then applies the provided `State` to it.  This is
 -- convenient for creating complicated structures.
-make :: Default def => State def a -> def
+make :: Message msg => State msg a -> msg
 make = modifyInState def
 
 -- | Allows one to modify a value in the 'State' monad.  Note that this is

proto-lens-discrimination/package.yaml

@@ -21,7 +21,6 @@ custom-setup:
 
 dependencies:
   - base >= 4.9 && < 4.12
-  - data-default >= 0.5 && < 0.8
   - bytestring == 0.10.*
   - contravariant >= 1.3 && < 1.5
   - containers >= 0.5 && < 0.6
@@ -54,10 +53,15 @@ tests:
     other-modules:
       - Proto.Enum
       - Proto.Enum_Fields
+      - Proto.Map
+      - Proto.Map_Fields
+
     # Manually list autogenerated modules, to avoid hpack
     # requiring `cabal-version: >= 2.0`.
     verbatim:
       - |
         autogen-modules:
           Proto.Enum
           Proto.Enum_Fields
+          Proto.Map
+          Proto.Map_Fields

proto-lens-discrimination/src/Data/ProtoLens/Discrimination.hs

@@ -15,7 +15,6 @@ module Data.ProtoLens.Discrimination
 import Data.Bits ((.|.), shift)
 import qualified Data.ByteString as B
 import qualified Data.ByteString.Unsafe as B (unsafeIndex)
-import Data.Default (Default(def))
 import Data.Foldable (foldMap)
 import Data.Functor.Contravariant
     ( Contravariant(contramap)
@@ -42,6 +41,8 @@ import Data.ProtoLens.Message
     ( FieldDescriptor(FieldDescriptor)
     , FieldTypeDescriptor(..)
     , FieldAccessor(..)
+    , Message
+    , def
     )
 
 -- | Sort values according to a Foldable of field descriptors.
@@ -126,7 +127,7 @@ discFieldSet discList disc32 disc64 discBS discInt =
 -- This should be identical to sorting @Map key value@s using the 'Ord' of
 -- @key@ and the 'Message' of value.
 discProtoMapAssocs
-    :: forall f entry key value. (Contravariant f, Default entry)
+    :: forall f entry key value. (Contravariant f, Message entry)
     => (forall a. f a -> f [a])
     -> f entry
     -> Lens' entry key

proto-lens-discrimination/tests/disc_test.hs

@@ -3,11 +3,10 @@
 module Main where
 
 import Control.Applicative ((<$>))
-import Data.Default (Default(def))
 import Data.Discrimination (Sort, runSort, sorting, sorting1)
 import Data.Int (Int32)
 import Data.List (sortBy)
-import Lens.Family2 ((&), (^.), (.~))
+import Lens.Family2 (Lens', (&), (^.), (.~))
 import Lens.Family2.Stock (_1, _2)
 import Test.HUnit ((@=?), Assertion)
 import Test.Framework (testGroup, defaultMain)
@@ -24,12 +23,15 @@ import Data.ProtoLens.Message
     , Message(fieldsByTextFormatName)
     , ScalarField(..)
     , MessageOrGroup(..)
+    , def
     )
 import Data.ProtoLens.Discrimination (discProtoMapAssocs)
 import Data.ProtoLens.Sort
 
 import Proto.Enum
 import Proto.Enum_Fields
+import Proto.Map
+import Proto.Map_Fields
 
 
 sortCompare :: Sort a -> a -> a -> Ordering
@@ -96,7 +98,9 @@ protoMapSortTest = testGroup "map"
           in compare x' y' == sortCompare c x' y'
     ]
   where
-    c = discProtoMapAssocs sorting1 sorting _1 _2
+    c = discProtoMapAssocs sorting1 sortingMessage
+            (key :: Lens' MapWrapper'IntMapEntry Int32)
+            value
 
 fieldSortTest :: Test
 fieldSortTest = testProperty "compares by field" $

proto-lens-discrimination/tests/map.proto

@@ -0,0 +1,8 @@
+syntax = "proto2";
+
+package map;
+
+message MapWrapper {
+    map<int32, int32> int_map = 1;
+}
+

proto-lens-protoc/src/Data/ProtoLens/Compiler/Generate.hs

@@ -114,7 +114,7 @@ generateModule modName imports syntaxType modifyImport definitions importedEnv s
     sharedImports = map (modifyImport . importSimple)
               [ "Prelude", "Data.Int", "Data.Word"
               , "Data.ProtoLens", "Data.ProtoLens.Message.Enum", "Data.ProtoLens.Service.Types"
-              , "Lens.Family2", "Lens.Family2.Unchecked", "Data.Default.Class"
+              , "Lens.Family2", "Lens.Family2.Unchecked"
               , "Data.Text",  "Data.Map", "Data.ByteString", "Data.ByteString.Char8"
               , "Lens.Labels", "Text.Read"
               ]
@@ -302,26 +302,8 @@ generateMessageDecls fieldModName syntaxType env protoName info =
     , let sym = promoteSymbol $ lensSymbol i
     ]
     ++
-    -- instance Data.Default.Class.Default Bar where
-    [ uncommented $ instDecl [] ("Data.Default.Class.Default" `ihApp` [dataType])
-        -- def = Bar { _Bar_foo = 0 }
-        [
-            [ match "def" []
-                $ recConstr (unQual dataName) $
-                      [ fieldUpdate (unQual $ haskellRecordFieldName $ plainFieldName f)
-                            (hsFieldDefault syntaxType env (fieldDescriptor f))
-                      | f <- messageFields info
-                      ] ++
-                      [ fieldUpdate (unQual $ haskellRecordFieldName $ oneofFieldName o)
-                            "Prelude.Nothing"
-                      | o <- messageOneofFields info
-                      ] ++
-                      [ fieldUpdate (unQual $ messageUnknownFields info)
-                            "[]"]
-            ]
-        ]
     -- instance Message.Message Bar where
-    , uncommented $ instDecl [] ("Data.ProtoLens.Message" `ihApp` [dataType])
+    [ uncommented $ instDecl [] ("Data.ProtoLens.Message" `ihApp` [dataType])
         $ messageInstance syntaxType env protoName info
     ]
   where
@@ -523,10 +505,6 @@ generateEnumDecls Proto3 info =
             "Data.ProtoLens.Message.Enum.messageEnumFromThenTo"
         ]
 
-    -- instance Data.Default.Class.Default Foo where
-    --   def = FirstEnumValue
-    , instDecl [] ("Data.Default.Class.Default" `ihApp` [dataType])
-        [[match "def" [] defaultCon]]
     -- instance Data.ProtoLens.FieldDefault Foo where
     --   fieldDefault = FirstEnumValue
     , instDecl [] ("Data.ProtoLens.FieldDefault" `ihApp` [dataType])
@@ -601,10 +579,6 @@ generateEnumDecls Proto2 info =
     [ dataDecl dataName
         [conDecl n [] | n <- constructorNames]
         $ deriving' ["Prelude.Show", "Prelude.Eq", "Prelude.Ord"]
-    -- instance Data.Default.Class.Default Foo where
-    --   def = FirstEnumValue
-    , instDecl [] ("Data.Default.Class.Default" `ihApp` [dataType])
-        [[match "def" [] defaultCon]]
     -- instance Data.ProtoLens.FieldDefault Foo where
     --   fieldDefault = FirstEnumValue
     , instDecl [] ("Data.ProtoLens.FieldDefault" `ihApp` [dataType])
@@ -949,10 +923,10 @@ hsFieldDefault syntaxType env fd
 
 hsFieldValueDefault :: Env QName -> FieldDescriptorProto -> Exp
 hsFieldValueDefault env fd = case fd ^. type' of
-    FieldDescriptorProto'TYPE_MESSAGE -> "Data.Default.Class.def"
-    FieldDescriptorProto'TYPE_GROUP -> "Data.Default.Class.def"
+    FieldDescriptorProto'TYPE_MESSAGE -> "Data.ProtoLens.defaultMessage"
+    FieldDescriptorProto'TYPE_GROUP -> "Data.ProtoLens.defaultMessage"
     FieldDescriptorProto'TYPE_ENUM
-        | T.null def -> "Data.Default.Class.def"
+        | T.null def -> "Data.ProtoLens.fieldDefault"
         | Enum e <- definedFieldType fd env
         , Just v <- List.lookup def [ (enumValueDescriptor v ^. name, enumValueName v)
                                     | v <- enumValues e
@@ -1047,6 +1021,19 @@ messageInstance syntaxType env protoName m =
           let' (map (fieldDescriptorVarBind $ messageName m) $ fields)
               $ "Data.Map.fromList" @@ list fieldsByTag ]
     , [ match "unknownFields" [] $ rawFieldAccessor (unQual $ messageUnknownFields m) ]
+    , [ match "defaultMessage" []
+           $ recConstr (unQual $ messageName m) $
+                  [ fieldUpdate (unQual $ haskellRecordFieldName $ plainFieldName f)
+                        (hsFieldDefault syntaxType env (fieldDescriptor f))
+                  | f <- messageFields m
+                  ] ++
+                  [ fieldUpdate (unQual $ haskellRecordFieldName $ oneofFieldName o)
+                        "Prelude.Nothing"
+                  | o <- messageOneofFields m
+                  ] ++
+                  [ fieldUpdate (unQual $ messageUnknownFields m)
+                        "[]"]
+      ]
     ]
   where
     fieldsByTag =

proto-lens-tests/package.yaml

@@ -22,7 +22,6 @@ dependencies:
     - QuickCheck
     - base
     - bytestring
-    - data-default-class
     - lens-family
     - pretty
     - proto-lens

proto-lens-tests/tests/imports_test.hs

@@ -4,7 +4,7 @@
 {-# LANGUAGE TypeApplications #-}
 module Main where
 
-import Data.Default.Class (Default(def))
+import Data.ProtoLens (Message, def)
 import Lens.Labels (Lens', view, set)
 import Test.Framework.Providers.HUnit (testCase)
 import Test.HUnit ((@=?))
@@ -30,7 +30,7 @@ main = testMain
 -- (In this test, we're checking that the sub-field type got imported
 -- correctly from another file.)
 testField
-    :: forall a b . (Default a, Default b, Eq b, Show b)
+    :: forall a b . (Message a, Message b, Eq b, Show b)
     => Lens' a b -> IO ()
 testField f = def @=? view f (set f def def)
 

proto-lens-tests/tests/labels_test.hs

@@ -8,8 +8,7 @@ import qualified Lens.Family2
 import qualified Lens.Family
 import Proto.Canonical (Test1, Test3)
 
-import Data.Default.Class (def)
-import Data.ProtoLens (build)
+import Data.ProtoLens (build, def)
 import Data.ProtoLens.TestUtil
 import Test.HUnit ((@?=))
 import Test.Framework.Providers.HUnit (testCase)

proto-lens-tests/tests/required_test.hs

@@ -7,8 +7,8 @@
 {-# LANGUAGE OverloadedStrings #-}
 module Main where
 
-import Data.Default.Class (def)
 import Data.Monoid ((<>))
+import Data.ProtoLens (def)
 import Lens.Family ((&), (.~))
 import Proto.Required (Foo)
 import Proto.Required_Fields (a, b)

proto-lens-tests/tests/unwrapped_test.hs

@@ -8,8 +8,7 @@ import Lens.Family ((&), (.~), (^.))
 import Lens.Labels.Unwrapped ()
 import Proto.Canonical (Test3)
 
-import Data.Default.Class (def)
-import Data.ProtoLens (build)
+import Data.ProtoLens (build, def)
 import Data.ProtoLens.TestUtil
 import Test.HUnit ((@?=))
 import Test.Framework.Providers.HUnit (testCase)

proto-lens/package.yaml

@@ -50,7 +50,6 @@ library:
     - bytestring == 0.10.*
     - containers == 0.5.*
     - deepseq == 1.4.*
-    - data-default-class >= 0.0 && < 0.2
     - lens-family == 1.2.*
     - lens-labels == 0.2.*
     - parsec == 3.1.*

proto-lens/src/Data/ProtoLens/Message.hs

@@ -30,7 +30,7 @@ module Data.ProtoLens.Message (
     FieldDefault(..),
     MessageEnum(..),
     -- * Building protocol buffers
-    Default(..),
+    def,
     build,
     -- * Proto registries
     Registry,
@@ -51,7 +51,6 @@ module Data.ProtoLens.Message (
     ) where
 
 import qualified Data.ByteString as B
-import Data.Default.Class
 import Data.Int
 import qualified Data.Map as Map
 import Data.Map (Map)
@@ -71,12 +70,19 @@ import Data.ProtoLens.Encoding.Wire
 -- | Every protocol buffer is an instance of 'Message'.  This class enables
 -- serialization by providing reflection of all of the fields that may be used
 -- by this type.
-class Default msg => Message msg where
+class Message msg where
     -- | A unique identifier for this type, of the format
     -- @"packagename.messagename"@.
     messageName :: Proxy msg -> T.Text
+
+    -- | A message with all fields set to their default values.
+    --
+    -- Satisfies @encodeMessage defaultMessage == ""@ and @decodeMessage "" == Right defaultMessage@.
+    defaultMessage :: msg
+
     -- | The fields of the proto, indexed by their (integer) tag.
     fieldsByTag :: Map Tag (FieldDescriptor msg)
+
     -- | This map is keyed by the name of the field used for text format protos.
     -- This is just the field name for every field except for group fields,
     -- which use their Message type name in text protos instead of their
@@ -85,6 +91,7 @@ class Default msg => Message msg where
     fieldsByTextFormatName :: Map String (FieldDescriptor msg)
     fieldsByTextFormatName =
         Map.fromList [(n, f) | f@(FieldDescriptor n _ _) <- allFields]
+
     -- | Access the unknown fields of a Message.
     unknownFields :: Lens' msg FieldSet
 
@@ -142,8 +149,9 @@ data WireDefault value where
 
 -- | A proto3 field type with an implicit default value.
 --
--- This is distinct from 'Data.Default' to avoid orphan instances, and because
--- 'Bool' doesn't necessarily have a good Default instance for general usage.
+-- This is distinct from, say, 'Data.Default' to avoid orphan instances, and
+-- because 'Bool' doesn't necessarily have a good Default instance for general
+-- usage.
 class FieldDefault value where
     fieldDefault :: value
 
@@ -236,14 +244,20 @@ class (Enum a, Bounded a) => MessageEnum a where
     -- no corresponding value was defined in the .proto file.
     readEnum :: String -> Maybe a
 
+-- | A message with all fields set to their default values.
+--
+-- This is an elided version 'defaultMessage'.
+def :: Message a => a
+def = defaultMessage
+
 -- | Utility function for building a message from a default value.
 -- For example:
 --
 -- > instance Default A where ...
 -- > x, y :: Lens' A Int
 -- > m :: A
 -- > m = build ((x .~ 5) . (y .~ 7))
-build :: Default a => (a -> a) -> a
+build :: Message a => (a -> a) -> a
 build = ($ def)
 
 -- | A helper lens for accessing optional fields.