Case-insensitive char/string comparison

[Eelis]

Data.Char provides functions to convert case, but does not provide a function to perform case-insensitive char/string comparison. Note that such functionality cannot be obtained by simply comparing for equality after performing toUpper/toLower, because some languages have things like multiple lowercase characters corresponding to the same uppercase character.

For example, in Greek, converting either σ or ς to uppercase yields Σ, while the inverse always yields σ. Consequently, the convert-all-to-uppercase approach would produce a false positive when comparing σ to ς, and the convert-all-to-lowercase approach would produce a false negative when comparing Σ to ς.

[Isaac Dupree]

Would/should a "case-insensitive compare" distinguish σ and ς? I think we need a standard Unicode-recommendation reference in order to determine what should be implemented. < http://www.unicode.org/unicode/reports/tr21/> might be a good starting-point; the recommendation seems to be "normalize it (in a more sophisticated way than just toUpper or toLower), then compare". If not done that way, c.i. equality is non-transitive! ("σ" /= "ς", "σ" == "Σ", "σ" == "ς")

[Isaac Dupree]

that last equation should read "Σ" == "ς"

[igloo]

It would be great if someone could make a proposal ( http://www.haskell.org/haskellwiki/Library_submissions) for this; looking at algorithms from the Unicode guys sounds like a good way to go.

[igloo]

Please follow the library submissions process for this:  http://www.haskell.org/haskellwiki/Library_submissions