Haskell/Solutions/Control structures

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case expressions

Exercises
Use a case statement to implement a fakeIf function which might be used as a replacement to the familiar if expressions.

fakeIf :: Bool -> a -> a -> a
fakeIf condition ifTrue ifFalse =
  case condition of
    True  -> ifTrue
    False -> ifFalse

Controlling actions, revisited

Exercises
Redo the "Haskell greeting" exercise in Simple input and output/Controlling actions, this time using a case statement. What does the following program print out? And why? main = do x <- getX putStrLn x getX = do return "My Shangri-La" return "beneath" return "the summer moon" return "I will" return "return" return "again"

1.

main = do
  putStrLn "Hello, what is your name?"
  name <- getLine
  case name of
      "Simon" -> greatlanguage
      "John"  -> greatlanguage
      "Phil"  -> greatlanguage
      "Koen"  -> putStrLn "I think debugging Haskell is fun."
      _       -> putStrLn "Sorry, I don't know you."
      where
      greatlanguage = putStrLn "I think Haskell is a great programming language."

2. Executing main will print "again". Remember that the value of a sequence of IO actions is the same as the value of the last action in the sequence. getX can also be written as:

getX =
  do return "again"

or even shorter, as:

getX = return "again"

As a result, x in the main function has the value "again", which will then be written to the screen.

Operators

Exercises

Lambdas are a nice way to avoid defining unnecessary separate functions. Convert the following let- or where-bindings to lambdas: map f xs where f x = x * 2 + 3 let f x y = read x + y in foldr f 1 xs Sections are just syntactic sugar for lambda operations. I.e. (+2) is equivalent to \x -> x + 2. What would the following sections 'desugar' to? What would be their types? (4+) (1 `elem`) (`notElem` "abc")

1.

• Substitute f to get map (\ x -> x * 2 + 3) xs
• Substitute f to get foldr (\ x y -> read x + y) 1 xs

2.

• (4+)
  • Becomes (\ x -> 4 + x)
  • Has type Num a => a -> a
• (1 `elem`)
  • Becomes(\ x -> 1 `elem` x)
  • Alternately written as (\ x -> elem 1 x)
  • Has type Num a :: a -> Bool
    • Note: The full type is (Foldable t, Eq a, Num a) => t a -> Bool but this has not been covered yet.
• (`notElem` "abc")
  • Becomes (\ x -> x `notElem` "abc")
  • Alternately written as (\ x -> notElem x "abc")
  • Has type Char -> Bool