I am trying to make sense of the relative indexing example used in this "Charting Patterns on Price history" paper: http://serv1.ist.psu.edu:8080/viewdoc/download;jsessionid=CC3DEF7277760C535FE3AB7C51A2BE90?doi=10.1.1.21.6892&rep=rep1&type=pdf In Section 3 it defines: type Indicator a = Bar → (Maybe a ) Indicator takes a bar b and returns an indicator value for that bar. .....bar is associated with five basic fields: high, low, open, close price, and transaction volume ...... It is very common while defining indicators to use past values of an indicator. To support this, we have a combina- tor (♯) which enables relative indexing into past data. Given a bar (time), while an indicator, say high , evaluates to the high price at the bar, high ♯ n yields the high price of n th previous bar. (♯) is defined as follows: (♯) :: Indicator a → Integer → Indicator a ind ♯ n = λ b . ind (b − n ) I can't figure out how (b-n) translates to n'th previous bar. Any ideas? daryoush