makeFilter :: (b -> b -> Bool) -> (a -> b) -> b -> a -> Bool makeFilter (==) proj expected = (expected ==) . proj makeEqFilter :: Eq b => (a -> b) -> b -> a -> Bool makeEqFilter = makeFilter (==) Then you have a foo: data Foo = Foo { fooA :: String, fooB :: Int } foos = [Foo "a" 1, Foo "b" 2] filter (makeEqFilter fooA 1) foos and so on. Though this is not really buying you all that much over filter ((1 ==) . fooA) foos Or for storing data Query a = Query String (a -> Bool) let myQuery = Query "Test if fst is 1" ((1 ==) . fst) ... filter myQuery foos Does this help? -Ross On Jan 23, 2009, at 4:20 PM, Dominic Espinosa wrote:
Novice question here. Sorry if the post is wordy.
In the following code (which doesn't actually compile as-is), I'm trying to generalize these 'make*Filter' functions into a single 'makeFilter' function. However, I can't get the types to work right.
Foo is a tuple type on which a large number of accessor functions are defined. All of them have type Foo -> Int, Foo -> String, or Foo -> [a] (so far).
I tried defining 'Query' using exsistential types instead, but had difficulty with 'escaped type variables' when I tried to write a generalized 'makeFilter' function.
The general point of makeFilter is to take as parameters a Query, a value, a comparison function, and then return a function (Foo -> Bool). This returned function takes as its argument an object of type Foo, applies the function 'q' to it, compares that value to 'val', and finally returns a Bool.
Later on in the program, a list of these filter functions is to be used with a list of Foo objects, to determine which Foo objects satisfy all of the filters.
Advice would be greatly appreciated.
--- code ---
data Query a = Query { query_identifier :: String, query_func :: (Foo -> a) }
makeIntFilter :: Query Int -> Int -> (Int -> Int -> Bool) -> (Foo -> Bool) makeIntFilter q val cmp = (\k -> val `cmp` (query_func q $ k))
makeStringFilter :: Query String -> String -> (String -> String -> Bool) -> (Foo -> Bool) makeStringFilter q val cmp = (\k -> val `cmp` (query_func q $ k))
-- ??? broken, and the 'cmp' argument is thrown away, which seems wrong makeMemberFilter :: Eq a => Query [a] -> a -> (a -> a -> a) -> (Foo -> Bool) makeMemberFilter q val cmp =(\k -> val `elem` (query_func q $ k))
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