Add rewrite rules for text conversion

Closes #212.

I had to add `stringToText` and `textToString` functions in order to
specify inlining pragma as mentioned
[here](https://gitlab.haskell.org/ghc/ghc/issues/12632).

Benchmarks without rewrite rule:

```
benchmarked toText/toString
time                 1.013 ms   (875.1 μs .. 1.148 ms)
                     0.819 R²   (0.691 R² .. 0.904 R²)
mean                 1.621 ms   (1.268 ms .. 2.619 ms)
std dev              2.134 ms   (331.2 μs .. 4.071 ms)
variance introduced by outliers: 97% (severely inflated)
```

And with rewrite rule:

```
benchmarked toText/toString
time                 186.1 μs   (183.3 μs .. 188.4 μs)
                     0.998 R²   (0.996 R² .. 0.999 R²)
mean                 189.4 μs   (187.7 μs .. 191.8 μs)
std dev              7.508 μs   (5.823 μs .. 10.36 μs)
variance introduced by outliers: 20% (moderately inflated)
```

benchmark/Main.hs

@@ -24,6 +24,7 @@ main = defaultMain
  , bgroupConcatMap
  , bgroupMember
  , bgroupFold
+ , bgroupTextConversion
  ]
 
 bgroupList :: forall a .
@@ -171,3 +172,8 @@ bgroupFold = do
  bgroup "foldl'" [ bench "flipped" $ nf flipFoldl' testList
  , bench "base" $ nf ghcFoldl' testList
  ]
+
+bgroupTextConversion :: Benchmark
+bgroupTextConversion =
+ let str = replicate 100000 'a'
+ in bench "toText/toString" $ whnf (length . toString . toText) str

src/Universum/String/Conversion.hs

@@ -124,15 +124,19 @@ instance ConvertUtf8 LT.Text LB.ByteString where
 class ToText a where
  toText :: a -> T.Text
 
-instance ToText String where
- toText = T.pack
-
 instance ToText T.Text where
  toText = id
 
 instance ToText LT.Text where
  toText = LT.toStrict
 
+instance ToText String where
+ toText = stringToText
+
+stringToText :: String -> Text
+stringToText = T.pack
+{-# NOINLINE [4] stringToText #-}
+
 -- | Type class for converting other strings to 'LT.Text'.
 class ToLText a where
  toLText :: a -> LT.Text
@@ -153,12 +157,22 @@ class ToString a where
 instance ToString String where
  toString = id
 
-instance ToString T.Text where
- toString = T.unpack
-
 instance ToString LT.Text where
  toString = LT.unpack
 
+instance ToString T.Text where
+ toString = textToString
+
+textToString :: Text -> String
+textToString = T.unpack
+{-# NOINLINE [4] textToString #-}
+
+-- This pattern may occur quite often because we tend to use 'Text'
+-- rather than 'String' in function signatures.
+{-# RULES
+"toString/toText" [~3] forall a. textToString (stringToText a) = a
+#-}
+
 -- | Polymorhpic version of 'Text.Read.readEither'.
 --
 -- >>> readEither @Text @Int "123"

test/Test/Universum/Property.hs

@@ -2,28 +2,32 @@ module Test.Universum.Property
  ( hedgehogTestTree
  ) where
 
-import Universum 
+import Universum
+import qualified Universum as U
 
 import Data.List (nub)
-import Hedgehog (Property, Gen, MonadGen, forAll, property, assert, (===))
-import Test.Tasty (testGroup, TestTree)
+import Hedgehog (Gen, MonadGen, Property, assert, forAll, property, (===))
+import Test.Tasty (TestTree, testGroup)
 import Test.Tasty.Hedgehog
 
-import qualified Universum as U
-import qualified Hedgehog.Gen as Gen
-import qualified Hedgehog.Range as Range
-import qualified Data.ByteString as B
-import qualified Data.ByteString.Lazy as LB
-import qualified Data.Text as T
-import qualified Data.Text.Lazy as LT
-
+import qualified Data.ByteString as B
+import qualified Data.ByteString.Lazy as LB
+import qualified Data.Text as T
+import qualified Data.Text.Lazy as LT
+import qualified Hedgehog.Gen as Gen
+import qualified Hedgehog.Range as Range
 
 
 hedgehogTestTree :: TestTree
-hedgehogTestTree = testGroup "Tests" [utfProps, listProps, boolMProps]
+hedgehogTestTree = testGroup "Tests" [stringProps, utfProps, listProps, boolMProps]
+
+stringProps :: TestTree
+stringProps = testGroup "String conversions"
+ [ testProperty "toString . toText = id" prop_StringToTextAndBack
+ ]
 
 utfProps :: TestTree
-utfProps = testGroup "utf8 conversion property tests" 
+utfProps = testGroup "utf8 conversion property tests"
  [ testProperty "String to ByteString invertible" prop_StringToBytes
  , testProperty "Text to ByteString invertible" prop_TextToBytes
  , testProperty "ByteString to Text or String invertible" prop_BytesTo
@@ -48,7 +52,12 @@ utf8Bytes = Gen.utf8 (Range.linear 0 10000) unicode'
 -- "\65534" fails, but this is from BU.toString
 -- > import qualified Data.ByteString.UTF8 as BU
 -- > BU.toString (BU.fromString "\65534") == "\65533"
--- > True 
+-- > True
+
+prop_StringToTextAndBack :: Property
+prop_StringToTextAndBack = property $ do
+ str <- forAll utf8String
+ toString (toText str) === str
 
 prop_StringToBytes :: Property
 prop_StringToBytes = property $ do
@@ -74,7 +83,7 @@ prop_BytesTo = property $ do
 -- ordNub
 
 listProps :: TestTree
-listProps = testGroup "list function property tests" 
+listProps = testGroup "list function property tests"
  [ testProperty "Hedgehog ordNub xs == nub xs" prop_ordNubCorrect
  , testProperty "Hedgehog hashNub xs == nub xs" prop_hashNubCorrect
  , testProperty "Hedgehog sortNub xs == sort $ nub xs" prop_sortNubCorrect
@@ -126,4 +135,3 @@ prop_orM :: Property
 prop_orM = property $ do
  bs <- forAll genBoolList
  U.orM (return <$> bs) === ((return $ U.or bs) :: U.Maybe U.Bool)
-