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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