add eitherReader

CHANGELOG.markdown

@@ -5,6 +5,8 @@
 
   * Added `instance (Field e a, ...) => Field e (Maybe a)` for generic environment parsing. (https://github.com/supki/envparse/pull/16)
 
+  * Added `eitherReader`. (https://github.com/supki/envparse/pull/24)
+
 0.5
 ===
 

src/Env.hs

@@ -59,6 +59,7 @@ module Env
   , var
   , Var
   , Reader
+  , eitherReader
   , str
   , char
   , nonempty

src/Env/Internal/Parser.hs

@@ -14,6 +14,7 @@ module Env.Internal.Parser
   , Var(..)
   , defaultVar
   , Reader
+  , eitherReader
   , str
   , char
   , nonempty
@@ -125,6 +126,12 @@ liftVarF =
 -- | An environment variable's value parser. Use @(<=<)@ and @(>=>)@ to combine these
 type Reader e a = String -> Either e a
 
+-- | Create a 'Reader' from a simple parser function
+eitherReader :: Error.AsUnread e => (String -> Either String a) -> Reader e a
+eitherReader f s = left (Error.unread . suffix) $ f s
+ where
+  suffix x = x <> ": " <> show s
+
 -- | Parse a particular variable from the environment
 --
 -- @