Published at Jul 13 2018
·
0 comments

Instructions

Test suite

Solution

Implement basic list operations.

In functional languages list operations like `length`

, `map`

, and
`reduce`

are very common. Implement a series of basic list operations,
without using existing functions.

For installation and learning resources, refer to the exercism help page.

To run the test suite, execute the following command:

```
stack test
```

```
No .cabal file found in directory
```

You are probably running an old stack version and need to upgrade it.

```
No compiler found, expected minor version match with...
Try running "stack setup" to install the correct GHC...
```

Just do as it says and it will download and install the correct compiler version:

```
stack setup
```

If you want to play with your solution in GHCi, just run the command:

```
stack ghci
```

The exercism/haskell repository on GitHub is the home for all of the Haskell exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

```
{-# LANGUAGE DeriveAnyClass #-}
import Control.Exception (Exception, throw, evaluate)
import Test.Hspec (Spec, describe, it, shouldBe, shouldThrow)
import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)
import Prelude hiding
( (++)
, concat
, filter
, foldr
, length
, map
, reverse
)
import ListOps
( (++)
, concat
, filter
, foldl'
, foldr
, length
, map
, reverse
)
data StrictException = StrictException deriving (Eq, Show, Exception)
main :: IO ()
main = hspecWith defaultConfig {configFastFail = True} specs
specs :: Spec
specs = do
let big = 100000 :: Int
describe "length" $ do
it "of empty list" $
length ([] :: [Int]) `shouldBe` 0
it "of non-empty list" $
length [1 .. 4 :: Int] `shouldBe` 4
-- Track-specific test
it "of large list" $
length [1 .. big :: Int] `shouldBe` big
describe "reverse" $ do
it "of empty list" $
reverse ([] :: [Int]) `shouldBe` []
it "of non-empty list" $
reverse [1 .. 100 :: Int] `shouldBe` [100 , 99 .. 1]
describe "map" $ do
it "of empty list" $
map (+1) ([] :: [Int]) `shouldBe` []
it "of non-empty list" $
map (+1) [1, 3 .. 7 :: Int] `shouldBe` [2, 4 .. 8]
describe "filter" $ do
it "of empty list" $
filter undefined ([] :: [Int]) `shouldBe` []
it "of normal list" $
filter odd [1 .. 4 :: Int] `shouldBe` [1, 3]
describe "foldl'" $ do
it "of empty list" $
foldl' (+) (0 :: Int) [] `shouldBe` 0
it "of non-empty list" $
foldl' (+) (-3) [1 .. 4 :: Int] `shouldBe` 7
-- Track-specific test
it "of huge list" $
foldl' (+) 0 [1 .. big] `shouldBe` big * (big + 1) `div` 2
it "with non-commutative function" $
foldl' (-) 10 [1 .. 4 :: Int] `shouldBe` 0
-- Track-specific test
it "is not just foldr . flip" $
foldl' (flip (:)) [] "asdf" `shouldBe` "fdsa"
-- Track-specific test
it "is accumulator-strict (use seq or BangPatterns)" $
evaluate (foldl' (flip const) () [throw StrictException, ()])
`shouldThrow` (== StrictException)
describe "foldr" $ do
it "of empty list" $
foldr (*) (2 :: Int) [] `shouldBe` 2
it "of non-empty list" $
foldr (+) 5 [1 .. 4 :: Int] `shouldBe` 15
it "with non-commutative function" $
foldr div 5 [2, 5 :: Int] `shouldBe` 2
-- Track-specific test
it "as id" $
foldr (:) [] [1 .. big] `shouldBe` [1 .. big]
-- Track-specific test
it "as append" $
foldr (:) [100 .. big] [1 .. 99] `shouldBe` [1 .. big]
describe "++" $ do
it "of empty lists" $
[] ++ ([] :: [Int]) `shouldBe` []
it "of empty and non-empty lists" $
[] ++ [1 .. 4 :: Int] `shouldBe` [1 .. 4]
it "of non-empty and empty lists" $
[1 .. 4 :: Int] ++ [] `shouldBe` [1 .. 4]
it "of non-empty lists" $
[1 .. 3] ++ [4, 5 :: Int] `shouldBe` [1 .. 5]
-- Track-specific test
it "of large lists" $
[1 .. big `div` 2] ++ [1 + big `div` 2 .. big] `shouldBe` [1 .. big]
describe "concat" $ do
it "of no lists" $
concat ([] :: [[Int]]) `shouldBe` []
it "of list of lists" $
concat [[1, 2], [3], [], [4, 5, 6 :: Int]] `shouldBe` [1 .. 6]
-- Track-specific test
it "of large list of small lists" $
concat (map (:[]) [1 .. big]) `shouldBe` [1 .. big]
```

```
module ListOps
( length
, reverse
, map
, filter
, foldr
, foldl'
, (++)
, concat
) where
import Prelude hiding
( length, reverse, map, filter, foldr, (++), concat )
foldl' :: (b -> a -> b) -> b -> [a] -> b
foldl' _ z [] = z
foldl' f z (x:xs) = z' `seq` foldl' f z' xs
where z' = f z x
foldr :: (a -> b -> b) -> b -> [a] -> b
foldr _ b [] = b
foldr f z (x:xs) = f x z'
where z' = foldr f z xs
length :: [a] -> Int
length = foldl' (flip (const succ)) 0
reverse :: [a] -> [a]
reverse = foldl' (flip (:)) []
map :: (a -> b) -> [a] -> [b]
map f = foldr go []
where go a bs = f a : bs
filter :: (a -> Bool) -> [a] -> [a]
filter f = foldr (go f) []
where
go f1 y ys
| f1 y = y : ys
| otherwise = ys
(++) :: [a] -> [a] -> [a]
(++) xs ys = foldr (:) ys xs
concat :: [[a]] -> [a]
concat = foldr (++) []
```

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

Level up your programming skills with 3,368 exercises across 50 languages, and insightful discussion with our volunteer team of welcoming mentors.
Exercism is
**100% free forever**.

## Community comments