[serokell#165] readMaybe type changing

Problem: `readMaybe` type isn't polymorphic.
One of key points in the design of `Universum` is to avoid `String` as much as possible.

Solution: changed type to `readMaybe :: (ToString a, Read b) => a -> Maybe b`

Author: DK318

CHANGES.md

@@ -15,6 +15,10 @@ Unreleased
 * [#182](https://github.com/serokell/universum/issues/182):
   Deprecate `microlens` and `microlens-mtl` dependencies.
 
+* [#165](https://github.com/serokell/universum/issues/165):
+  Change the type of `readMaybe` from `readMaybe :: Read a => String -> Maybe a`
+  to it's polymorphic version `readMaybe :: forall b a. (ToString a, Read b) => a -> Maybe b`.
+
 1.7.3
 =====
 

src/Universum/String/Conversion.hs

@@ -25,12 +25,14 @@ module Universum.String.Conversion
 
          -- * Show and read functions
        , readEither
+       , readMaybe
        , show
        ) where
 
 import Data.Bifunctor (first)
 import Data.Either (Either)
 import Data.Function (id, (.))
+import Data.Maybe (Maybe)
 import Data.String (String)
 import qualified Data.Text.Internal as T
 import qualified Data.Text.Internal.Fusion.Common as TF
@@ -47,7 +49,7 @@ import qualified Data.Text.Encoding as T
 import qualified Data.Text.Encoding.Error as T
 import qualified Data.Text.Lazy as LT
 import qualified Data.Text.Lazy.Encoding as LT
-import qualified Text.Read (readEither)
+import qualified Text.Read (readEither, readMaybe)
 
 import qualified GHC.Show as Show (Show (show))
 
@@ -293,6 +295,15 @@ be safe to collapse.
 readEither :: (ToString a, Read b) => a -> Either Text b
 readEither = first toText . Text.Read.readEither . toString
 
+-- | Polymorhpic version of 'Text.Read.readMaybe'.
+--
+-- >>> readMaybe @Int @Text "123"
+-- Just 123
+-- >>> readMaybe @Int @Text "aa"
+-- Nothing
+readMaybe :: forall b a. (ToString a, Read b) => a -> Maybe b
+readMaybe = Text.Read.readMaybe . toString
+
 -- | Generalized version of 'Prelude.show'.
 show :: forall b a . (Show.Show a, IsString b) => a -> b
 show x = fromString (Show.show x)

src/Universum/String/Reexport.hs

@@ -24,4 +24,4 @@ import Data.Text.Encoding (decodeUtf8', decodeUtf8With)
 import Data.Text.Encoding.Error (OnDecodeError, OnError, UnicodeException, lenientDecode,
                                  strictDecode)
 import Data.Text.Lazy (fromStrict, toStrict)
-import Text.Read (Read, readMaybe, reads)
+import Text.Read (Read, reads)