If I'm understanding you, the answer is 'you can safely call encodeString on ASCII text, and UTF text, but you cannot on ISO8859-1 & ASCII Extended'. So we can either default to calling encodeString, checking whether it's ISO/Extended (and not calling encodeString if True); or we can default to not calling encodeString, and enabling it if a check for UTF returns true.
I guess since Alexey has already provided a check for UTF, then we should probably use the latter strategy.
The only problem are user with one byte encodings. encodeString can be safely called on string which contain only ASCII characters. For ASCII input encodeString == id. So nothing will change for ASCII. Users with UTF8 locale will get nicely encoded strings. Users will get garbage. But they have it anyway, it only will look different. So I think it sound solution to wrap everything into encodeString. It is not The Right Way To Do Things. It's only a workaround... still better than nothing. It's job for standard libraries... not for software developers. For more information on issue read below. There is no such thing as [some encoding] encoded string in haskell (most of the time). Strings in haskell _are_ unicode. Char is valid unicode code point. Not byte, word32, etc. It's fairly abstract code point. Usually they contain only ASCII characters but they unicode nevertheless. Char is represented somehow but one shouldn't bother about it most of the time. Problems arise when char passed to outside world. World understand sequences of bytes so strings must be encoded somehow. Standard library uses very simple method: (\c -> c .&. 0xff). Every character translated to one byte. Simple but works only for ASCII (and maybe latin-1). Because of that behavior all that encodeStrings are needed. Some examples to illustrate above: ы U+044B Name: CYRILLIC SMALL LETTER YERU Prelude> fromEnum $ 'ы' 1099 -- (1099 == 0x44b) Prelude> putStrLn . encodeString $ [toEnum 0x44b] ы Prelude> putStrLn $ [toEnum 0x44b] K Prelude> putStrLn . encodeString $ [toEnum $ 0xff .&. 0x44b] K P.S. It is not safe to call encodeString on UTF8 encoded string. No encoding. Pass string as it is Prelude> putStrLn "Ну что тут с уникодом?" C GB> BCB A C=8:>4> putStrLn . encodeString $ "Ну что тут с уникодом?" Ну что тут с уникодом? Encode string which already UTF8 encoded. Prelude> putStrLn . encodeString . encodeString $ "Ну что тут с уникодом?" ÐÑ ÑÑо ÑÑÑ Ñ Ñникодом?