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to Accumulate in the Haskell Track

Published at Jul 13 2018 · 1 comment
Test suite

Implement the accumulate operation, which, given a collection and an operation to perform on each element of the collection, returns a new collection containing the result of applying that operation to each element of the input collection.

Given the collection of numbers:

  • 1, 2, 3, 4, 5

And the operation:

  • square a number (x => x * x)

Your code should be able to produce the collection of squares:

  • 1, 4, 9, 16, 25

Check out the test suite to see the expected function signature.


Keep your hands off that collect/map/fmap/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.

Getting Started

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

Running the tests

To run the test suite, execute the following command:

stack test

If you get an error message like this...

No .cabal file found in directory

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

Otherwise, if you get an error message like this...

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

Running GHCi

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

stack ghci

Feedback, Issues, Pull Requests

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!


Conversation with James Edward Gray II https://twitter.com/jeg2

Submitting Incomplete Solutions

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


import Data.Char         (toUpper)
import Test.Hspec        (Spec, it, shouldBe)
import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)

import Accumulate (accumulate)

main :: IO ()
main = hspecWith defaultConfig {configFastFail = True} specs

specs :: Spec
specs = do

    let square x = x * x :: Int

    it "empty accumulation" $
      accumulate square []
      `shouldBe` []

    it "accumulate squares" $
      accumulate square [1, 2, 3]
      `shouldBe` [1, 4, 9]

    it "accumulate upcases" $
      accumulate (map toUpper) ["hello", "world"]
      `shouldBe` ["HELLO", "WORLD"]

    it "accumulate reversed strings" $
      accumulate reverse ["the", "quick", "brown", "fox", "etc"]
      `shouldBe` ["eht", "kciuq", "nworb", "xof", "cte"]

    it "accumulate recursively" $
      accumulate (\c -> accumulate ((c:) . show) ([1, 2, 3] :: [Int])) "abc"
      `shouldBe` [["a1", "a2", "a3"], ["b1", "b2", "b3"], ["c1", "c2", "c3"]]

    it "accumulate non-strict" $
      take 1 (accumulate id ("nice work!" : error "accumulate should be even lazier, don't use reverse!"))
      `shouldBe` ["nice work!"]
module Accumulate (accumulate) where

accumulate :: (a -> b) -> [a] -> [b]
accumulate _ [] = []
accumulate f (x:xs) = f x : accumulate f xs

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Avatar of agbell

This is map.

What can you learn from this solution?

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?