diff --git a/Control/GroupWith.hs b/Control/GroupWith.hs new file mode 100644 --- /dev/null +++ b/Control/GroupWith.hs @@ -0,0 +1,67 @@ + +----------------------------------------------------------------------------- +-- | +-- Module : Control.GroupWiths +-- Copyright : (c) Uli Köhler 2014 +-- License : Apache License v2.0 +-- Maintainer : ukoehler@techoverflow.net +-- Stability : provisional +-- Portability : portable +-- +-- A collection of grouping utility functions. +-- For a given function that assigns a key to objects, +-- provides functions that group said objects into a multimap +-- by said key. +-- +-- This can be used similarly to the SQL GROUP BY statement. +-- +-- Provides a more flexible approach to GHC.Exts.groupWith +-- +-- > groupWith (take 1) ["a","ab","bc"] == Map.fromList [("a",["a","ab"]), ("b",["bc"])] +-- +----------------------------------------------------------------------------- +module Control.GroupWith( + MultiMap, + groupWith, + groupWithMultiple, + groupWithUsing + ) where + +import Data.Map (Map) +import qualified Data.Map as Map + +type MultiMap a b = Map a [b] + +-- | Group values in a list by a key, generated +-- by a given function. The resulting map contains +-- for each generated key the values (from the given list) +-- that yielded said key by applying the function on said value. +groupWith :: (Ord b) => + (a -> b) -- ^ The function used to map a list value to its key + -> [a] -- ^ The list to be grouped + -> MultiMap b a -- ^ The resulting key --> value multimap +groupWith f xs = Map.fromListWith (++) [(f x, [x]) | x <- xs] + +-- | Like groupWith, but the identifier-generating function +-- may generate multiple keys for each value (or none at all). +-- The corresponding value from the original list will be placed +-- in the identifier-corresponding map entry for each generated +-- identifier. +-- Note that values are added to the +groupWithMultiple :: (Ord b) => + (a -> [b]) -- ^ The function used to map a list value to its keys + -> [a] -- ^ The list to be grouped + -> MultiMap b a -- ^ The resulting map +groupWithMultiple f xs = + let identifiers x = [(val, [x]) | val <- f x] + in Map.fromListWith (++) $ concat [identifiers x | x <- xs] + +-- | Like groupWith, but uses a custom combinator function +groupWithUsing :: (Ord b) => + (a -> c) -- ^ Transformer function used to map a value to the resulting type + -> (c -> c -> c) -- ^ The combinator used to combine an existing value + -- for a given key with a new value + -> (a -> b) -- ^ The function used to map a list value to its key + -> [a] -- ^ The list to be grouped + -> Map b c -- ^ The resulting key --> transformed value map +groupWithUsing t c f xs = Map.fromListWith c $ map (\v -> (f v, t v)) xs diff --git a/GroupWithTest.hs b/GroupWithTest.hs new file mode 100644 --- /dev/null +++ b/GroupWithTest.hs @@ -0,0 +1,78 @@ +import Test.Hspec +import Test.QuickCheck +import Control.Exception (evaluate) +import Control.Monad +import Control.GroupWith +import Data.Map (Map) +import Data.List (sort) +import qualified Data.Map as Map + +multimapElemEq :: (Eq k, Ord v) => (k,[v]) -> (k,[v]) -> Bool +multimapElemEq (k1,v1) (k2,v2) = (k1 == k2) && (sort v1 == sort v2) + +-- Check if two multimap-representing lists are equal, +-- disregarding value list order +multimapListEq :: (Ord a, Ord b, Eq a, Eq b) => [(a,[b])] -> [(a,[b])] -> Bool +multimapListEq xs ys = + let zipped = zipWith multimapElemEq xs ys + in and zipped + +multimapEq :: (Ord a, Ord b, Eq a, Eq b) => Map a [b] -> Map a [b] -> Bool +multimapEq x y = + multimapListEq (Map.toList x) (Map.toList y) + +multimapTupleEq :: (Ord a, Ord b, Eq a, Eq b) => (Map a [b], Map a [b]) -> Bool +multimapTupleEq (x,y) = multimapEq x y + +main :: IO () +main = hspec $ do + describe "groupWith" $ do + it "should group a simple value correctly" $ + let data_ = ["a","abc","ab","bc"] + fn = take 1 + ref = Map.fromList [("a",["a","abc","ab"]),("b",["bc"])] + result = groupWith fn data_ + in (result, ref) `shouldSatisfy` multimapTupleEq + it "should return an empty map when given an empty list" $ + -- We need to specialize here, because it's ⊥ + let fn = error "This function should never be called" :: Int -> Int + data_ = [] :: [Int] + result = groupWith fn data_ + in result `shouldSatisfy` ((==) 0 . Map.size) + -- Fuzzing, couldn't get this to compile properly yet + -- it "should not crash for any input string list" $ + -- property $ (\d -> groupWith (take 1) d `seq` True) + describe "groupWithUsing" $ do + it "should return an empty map when given an empty list" $ + -- We need to specialize here, because it's ⊥ + let fn = error "This function should never be called" :: Int -> Int + data_ = [] :: [Int] + result = groupWith fn data_ + in result `shouldSatisfy` ((==) 0 . Map.size) + it "should be usable for counting" $ + -- Instead of building up lists, we count the number of elements + let t n = 1 -- For each x, count 1 + c = (+) -- Sum up the counts + fn = take 1 + data_ = ["a","abc","ab","bc"] + ref = Map.fromList [("a",3),("b",1)] :: Map String Int + result = groupWithUsing t c fn data_ + in result `shouldBe` ref + describe "groupWithMultiple" $ do + it "should group correctly given a simple list" $ + let data_ = ["a","abc","ab","bc"] + fn x = [take 1 x, take 2 x] + -- Note the multiple "a"s in the first line: + -- one from `take 1`, one from `take 2` + ref = Map.fromList [("a",["a","a","abc","ab"]), + ("ab",["ab","abc"]), + ("b",["bc"]), + ("bc",["bc"])] + result = groupWithMultiple fn data_ + in (result, ref) `shouldSatisfy` multimapTupleEq + it "should return an empty map when given an empty list" $ + -- We need to specialize here, because it's ⊥ + let fn = error "This function should never be called" :: Int -> [Int] + data_ = [] :: [Int] + result = groupWithMultiple fn data_ + in result `shouldSatisfy` ((==) 0 . 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