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composing projections



I'd like to compose projections. 
For example, given a list of tuples of lists I want to map a function that
finds the length of the first lists in each tuple:

map ((length o proj1) my_tuple)

But this generates the error: "Projection expressions may only appear as
operators of an application."

Can anyone suggest a way to accomplish this?

thanks,

Ben Kleinman
bkleinman@writeme.com
http://www.dcs.ed.ac.uk/home/bkk