[Haskell-cafe] Generalizing a filter-making function

[Ross Mellgren]

Parsec is an awesome piece of software. Throw away anything you might  
know from writing parsers in something else -- writing parsers with  
parsec is so succinct you can use it most anytime without feeling like  
you're resorting to it.

-Ross

On Jan 23, 2009, at 11:26 PM, Dominic Espinosa wrote:

> Thanks, that does help. I see I was unnecessarily passing a function
> parameter (as a newcomer to Haskell, I tend to forget how to properly
> use composition).
>
> The reason to do these maneuvers is that the particular accessor
> function to use is being parsed from user input. The main problem is
> along the lines of: given a user input string such as "s < 5, t  
> 'baz'",
> return a list of all Foos f such that (s f < 5) and ('baz' `elem` $  
> t f).
>
> Someone else tipped me off about Parsec's expression parsing  
> support, so
> I need to look at that as well. The expressions to be parsed are very
> minimal, however, so I'm not sure it's entirely necessary.
>
> Thanks again.
>
> On Fri, Jan 23, 2009 at 05:39:07PM -0500, Ross Mellgren wrote:
>> 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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>>