haskell - Get tuple from list and use it as Map key -

i'm haskell newbie , i'm trying use map.fromlist. have defined following data types:

type no = string data arco = arco { de :: no                  , para :: no                  , custo :: float                  } deriving (show, ord, eq) 

i have list of arco , want map them, using (de, para) tuple key, , custo value. how can achieve that?

thanks

i think achieve using folding functions or list mapping first convert list of arco values simple list, , using map.fromlist build map list of key/value pairs. along lines of (not tested):

map.fromlist (foldr (\x res -> ((de x, para x), custo x):res) [] yourarcolist) 

or map:

map.fromlist $ map (\x -> ((de x, para x), custo x)) yourarcolist 

or list comprehension:

[((de x, para x), custo x) | x <- yourarcolist] 

de, para, , cusot functions automatically created haskell can extract field values type values.