Add IO-free variants of some functions

Fixes #4

System/OsString/Common.hs

@@ -33,7 +33,13 @@ module System.OsString.MODULE_NAME
   , unsafeEncodeUtf
   , encodeWith
   , encodeFS
+#ifdef WINDOWS
+  , fromString
+#endif
   , fromBytes
+#ifndef WINDOWS
+  , fromBytestring
+#endif
   , pstr
   , singleton
   , empty
@@ -254,6 +260,18 @@ encodeFS = fmap WindowsString . encodeWithBaseWindows
 encodeFS = fmap PosixString . encodeWithBasePosix
 #endif
 
+#ifdef WINDOWS
+-- | Like 'encodeFS', but not in IO.
+--
+-- 'encodeFS' was designed to have a symmetric type signature
+-- on unix and windows, but morally the function has no IO effects on windows,
+-- so we provide this variant without breaking existing API.
+--
+-- This function does not exist on unix.
+fromString :: String -> WindowsString
+fromString = unsafePerformIO . fmap WindowsString . encodeWithBaseWindows
+#endif
+
 
 #ifdef WINDOWS_DOC
 -- | Partial unicode friendly decoding.
@@ -346,6 +364,18 @@ fromBytes bs =
 fromBytes = pure . PosixString . BSP.toShort
 #endif
 
+#ifndef WINDOWS
+-- | Like 'fromBytes', but not in IO.
+--
+-- 'fromBytes' was designed to have a symmetric type signature
+-- on unix and windows, but morally the function has no IO effects on unix,
+-- so we provide this variant without breaking existing API.
+--
+-- This function does not exist on windows.
+fromBytestring :: ByteString -> PosixString
+fromBytestring = PosixString . BSP.toShort
+#endif
+
 
 #ifdef WINDOWS_DOC
 -- | QuasiQuote a 'WindowsString'. This accepts Unicode characters