[#286] Consistent type for 'readMaybe' and 'readEither' (#296)

* [#286] Consistent type for 'readMaybe' and 'readEither' Resolves #286 * Update CHANGELOG.md Co-authored-by: Dmitrii Kovanikov <[email protected]> Co-authored-by: Dmitrii Kovanikov <[email protected]>
kowainik · May 14, 2020 · 7eb0ce0 · 7eb0ce0
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -64,6 +64,25 @@ The changelog is available [on GitHub][2].
   `Relude.List.Reexport`.
 * [#293](https://github.com/kowainik/relude/issues/293):
   Add `memptyIfFalse` and `memptyIfTrue` functions.
+* [#286](https://github.com/kowainik/relude/issues/286):
+  __Breaking change:__ `readEither` is not polymorphic over the first argument
+  anymore. Now it takes `String`.
+
+  __Migration rules:__ Use one of the conversion function from the
+  `Relude.String.Conversion` module to covert your old input value into
+  `String`.
+
+  For example, if you had
+
+  ```haskell
+  readEither @Text @Int myText
+  ```
+
+  Now it should become:
+
+  ```haskell
+  readEither @Int (toString myText)
+  ```
 
 ## 0.6.0.0 — Oct 30, 2019
 

diff --git a/src/Relude/String.hs b/src/Relude/String.hs
@@ -55,4 +55,5 @@ Reexport data types and functions to work with 'Text', 'ByteString',
 -}
 {- $conversion
 Conversion functions between 'Text', 'String', 'ByteString'.
+Also some read|show helper functions.
 -}
diff --git a/src/Relude/String/Conversion.hs b/src/Relude/String/Conversion.hs
@@ -482,15 +482,16 @@ type family EncodingError
         ':<>: 'Text " -> Either UnicodeException " ':<>: 'Text to
         )
 
-{- | Polymorhpic version of 'Text.Read.readEither'.
+{- | Version of 'Text.Read.readEither' that returns 'Text' in case of the parse
+error.
 
->>> readEither @Text @Int "123"
+>>> readEither @Int "123"
 Right 123
->>> readEither @Text @Int "aa"
+>>> readEither @Int "aa"
 Left "Prelude.read: no parse"
 -}
-readEither :: (ToString a, Read b) => a -> Either Text b
-readEither = first toText . Text.Read.readEither . toString
+readEither :: (Read a) => String -> Either Text a
+readEither = first toText . Text.Read.readEither
 {-# INLINEABLE readEither #-}
 
 {- | Generalized version of 'Prelude.show'. Unlike 'Prelude.show' this function