I hate to have to disagree with this site, but I really don't think that
Halle and
Hallie are forms of
Hayley. They have a different pronunciation from
Hayley (at least for most in the USA), and the name
Hallie was frequently used in the 19th century years before there were any noticeable number of girls named
Hayley.
Hallie is almost assuredly originally a pet form of
Harriet, just as
Hal was originally a pet form of
Harry or
Henry.
For examples of women officially named
Harriet but called
Hallie see:
http://www.askart.com/askart/i/harriett_hallie_hyde_irwin/harriett_hallie_hyde_irwin.aspx
http://poslfit.homeip.net/cgi-bin/genea2.pl?id=11938
http://home.netcom.com/~mohlerl/photos/lloyd_n_hallie.htm
Examples of women named
Hallie born in the 19th century include best-selling novelist
Hallie Erminie Rives, and
Hallie Q. Brown, a famous African-American activist:
http://www.aaregistry.com/african_american_history/744/A_Black_voice_for_womens__issues_Hallie_Q_Brown