I am guessing this is how the data got accumulated: in you my_store, the function stored is like \w -> if w == 'a' then 3 else if w == 'b' then 5 else if w == 'a' then 4 else (\w -> 0 ) w 'a' is stored twice here, ( maybe compiler will optimize it out but basically you see the lasted 'a' mapping value ) -----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org]On Behalf Of Petter Egesund Sent: Saturday, October 04, 2003 4:48 PM To: haskell-cafe@haskell.org Subject: Why does this work - haskell mysteries? Hi; the proof of the pudding does lies in the eating... but I still wonder why this code is working (it is taken from the book "The Craft of functional programming"). The program connects a variable-name to value. The fun initial gives the initial state, update sets a variable & value reads a value). I evaluate value my_store 'b' to 5 and value my_store 'a' to 3 as expected from the text in the book. But I can't see what is happening here. The book has a parallel example where the data is held in a list, and this version is easy to follow, but this trick with storing a lambda-function inside a newtype beats me. The problem is that I do not understand where the accumulated data is stored (not in a list - it seems like something like a chain of functions which can be pattern-matched, but I am not sure). And why does not the lambda-function (\w -> if v==w then n else sto w) start a endless loop? (This is not homework - I am a programmer who is curious about Haskell!) Any clues, anyone? Cheers, Petter -- Var is the type of variables. type Var = Char newtype Store = Sto (Var -> Int) -- initial :: Store initial = Sto (\v -> 0) value :: Store -> Var -> Int value (Sto sto) v = sto v update :: Store -> Var -> Int -> Store update (Sto sto) v n = Sto (\w -> if v==w then n else sto w) -- testit -- my_store = update (update (update initial 'a' 4) 'b' 5) 'a' 3) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe