Make all exercises not compile to start (#25)

Author: jhb563

README.md

@@ -37,7 +37,7 @@ Within the [exercises directory](https://github.com/MondayMorningHaskell/haskell
 
 This will run the [`Types1`](https://github.com/MondayMorningHaskell/haskellings/blob/master/exercises/basics/Types1.hs) exercise.
 
-Initially, exercises will not compile. You'll want to open the file, read the information at the top, and then fill in the code as indicated by the `TODO` comments.
+Initially, exercises will not compile. You'll want to open the file, read the information at the top, and then fill in the code as indicated by the `TODO` comments. You'll usually be replacing the question marks `???` with your function implementations and type declarations.
 
 You can re-run the exercise, and the output will let you know when you're done!
 
@@ -48,6 +48,26 @@ If you're stuck, each exercise also has a hint you can see by running `haskellin
 Fill in appropriate type signatures for the expressions at the bottom.
 ```
 
+Sometimes you might want to compile and run code you've written for one function without filling in the rest. When this happens, you can usually replace the question marks `???` for function implementations with the phrase `undefined`. Using `undefined` lets your code compile, though that particular function won't run:
+
+```haskell
+-- BEFORE:
+subtract7 :: Int -> Int
+subtract7 = ???
+
+multiplyBy7 :: Int -> Int
+multiplyBy7 = ???
+
+-- AFTER:
+subtract7 :: Int -> Int
+subtract7 x = x - 7
+
+multiplyBy7 :: Int -> Int
+multiplyBy7 = undefined
+```
+
+For example, after making the above change, you'll be able to run the tests for the `subtract7` function. You'll still see errors while running the tests for `multiplyBy7`, but they will compile. Sometimes the implementation will start as `undefined` rather than `???`, in which case it's still your job to replace it!
+
 ## Using the Watcher
 
 The simplest way to ensure that you're doing exercises _in order_ is to use the Watcher. You can invoke this with the command `haskellings watch`.

exercises/functions/Functions2.hs

@@ -40,16 +40,16 @@ Here are a few examples:
 -- Add the heads of the two inputs together. Return this as the first element
 -- of the tuple. Then you should also return the tail of each list.
 addHeads :: ([Int], [Int]) -> (Int, [Int], [Int])
-addHeads = undefined
+addHeads = ???
 
 -- Produce a new tuple containing the three *second* elements of each of
 -- the input tuples.
 takeSeconds :: (Int, Int) -> (Int, Int) -> (Int, Int) -> (Int, Int, Int)
-takeSeconds = undefined
+takeSeconds = ???
 
 -- Capitalize each of the three characters
 capitalize :: (Char, Char, Char) -> (Char, Char, Char)
-capitalize = undefined
+capitalize = ???
 
 -- Testing Code, You can ignore this:
 main :: IO ()

exercises/functions/Functions3.hs

@@ -30,7 +30,7 @@ multiplyBy3Add5 = multiplyBy3AndAdd 5
 -- Then multiply the second elements together as well.
 -- Add the results.
 multiplyAndAdd :: (Int, Int) -> (Int, Int) -> Int
-multiplyAndAdd = undefined
+multiplyAndAdd = ???
 
 -- Take a tuple of two Ints.
 -- Multiply the first value by 3 and the second by 4. Then add the results.

exercises/functions/Functions5.hs

@@ -41,23 +41,23 @@ the front of a list:
 -- This should take any boolean value and return True!
 -- (There are two easy ways to do this, try both!)
 trueSink :: Bool -> Bool
-trueSink = undefined
+trueSink = ???
 
 -- This should take any boolean value and return False!
 falseSink :: Bool -> Bool
-falseSink = undefined
+falseSink = ???
 
 -- Return True if and only if all three inputs are true.
 tripleAnd :: Bool -> Bool -> Bool -> Bool
-tripleAnd = undefined
+tripleAnd = ???
 
 -- Take two lists. Remove the first element from each list and add them together.
 -- The first output should be like the first list except with this new sum at the
 -- start instead of the original value. The second output should be the tail of the
 -- original second input.
 -- addToFirstList [1, 2] [9, 8] = ([10, 2], [8])
 addToFirstList :: [Int] -> [Int] -> ([Int], [Int])
-addToFirstList = undefined
+addToFirstList = ???
 
 -- Testing Code
 main :: IO ()

exercises/functions/Functions6.hs

@@ -36,11 +36,11 @@ doubleList xs = map double xs
 
 -- Flip the boolean value of each input
 flipBools :: [Bool] -> [Bool]
-flipBools input = undefined
+flipBools input = ???
 
 -- Uppercase all the letters in this word!
 capitalizeWord :: String -> String
-capitalizeWord input = undefined
+capitalizeWord input = ???
 
 -- TODO: Create your own higher order function, doubleAndApply.
 -- The input function should take a single integer

exercises/lists/Lists1.hs

@@ -60,15 +60,15 @@ foldr min 9999 [5, 7, 2, 9, 1]
 -- higher order helper functions instead!
 
 addMod3Is2 :: [Int] -> [Int]
-addMod3Is2 = undefined
+addMod3Is2 = ???
 
 sumList :: [Int] -> Int
-sumList = undefined
+sumList = ???
 
 -- Return true if every element is 'True'!
 -- (The empty list should return 'True'!)
 allTrue :: [Bool] -> Bool
-allTrue bs = undefined
+allTrue bs = ???
 
 main :: IO ()
 main = defaultMain $ testGroup "Lists1" $

exercises/lists/Lists2.hs

@@ -49,11 +49,11 @@ cycle [1, 2, 3, 4] -- < [1, 2, 3, 4, 1, 2, 3, 4, 1 ...]
 -- Given a particular number, give a list of the numbers starting
 -- there and counting down by 5 until you get to 0.
 countdownBy5 :: Word -> [Word]
-countdownBy5 = undefined
+countdownBy5 = ???
 
 -- Given a particular Int, create a list with 100 copies of it.
 give100 :: Int -> [Int]
-give100 = undefined
+give100 = ???
 
 main :: IO ()
 main = defaultMain $ testGroup "Lists2" $

exercises/lists/Lists3.hs

@@ -50,12 +50,12 @@ shortWords = ["HELLO", "BYE", "CIAO"]
 
 -- Implement addMod3Is2, except now use a list comprehension.
 addMod3Is2 :: [Int] -> [Int]
-addMod3Is2 = undefined
+addMod3Is2 = ???
 
 -- Take every pairwise product of the numbers, as long as their
 -- sum is less than 30.
 smallPairwiseProducts :: [Int] -> [Int] -> [Int]
-smallPairwiseProducts = undefined
+smallPairwiseProducts = ???
 
 main :: IO ()
 main = defaultMain $ testGroup "Lists3" $

exercises/lists/Lists4.hs

@@ -27,13 +27,13 @@ zip [1, 2, 3] ["Hi", "Bye"] = [(1, "Hi"), (2, "Bye")]
 -- return tuples containing the sum, product, and difference of each pair.
 -- sumProductDifference [4, 3] [1, 2] = [(5, 4, 3), (5, 6, 1)]
 sumProductDifference :: [Int] -> [Int] -> [(Int, Int, Int)]
-sumProductDifference = undefined
+sumProductDifference = ???
 
 -- Given a list of strings, make a new list where each String has been
 -- appended with its index within the list.
 -- addIndices ["Hello", "Bye"] = ["Hello0", "Bye1"]
 addIndices :: [String] -> [String]
-addIndices = undefined
+addIndices = ???
 
 main :: IO ()
 main = defaultMain $ testGroup "Lists4" $

exercises/monads/Applicatives.hs

@@ -65,18 +65,18 @@ safeSquareRoot x = if x < 0 then Nothing else Just (sqrt x)
 -- Return the sum of the square roots of the two inputs, using
 -- 'safeSquareRoot' to check for negatives.
 sumOfSquareRoots :: Double -> Double -> Maybe Double
-sumOfSquareRoots = undefined
+sumOfSquareRoots = ???
 
 -- Generate all combinations of sums between the first list and the second list.
 -- (avoid using a list comprehension)
 generateSums :: [Int] -> [Int] -> [Int]
-generateSums = undefined
+generateSums = ???
 
 -- Given a list of operations and two lists, generate all combinations of
 -- those operations with each pair of numbers from the two lists.
 -- generateAllResults [(+), (*)] [1, 2] [3, 4] -> [4, 5, 5, 6, 3, 4, 6, 8]
 generateAllResults :: [Int -> Int -> Int] -> [Int] -> [Int] -> [Int]
-generateAllResults = undefined
+generateAllResults = ???
 
 main :: IO ()
 main = defaultMain $ testGroup "Applicatives" $