closures.hs

inc :: Num a => a -> a
inc n = n + 1

double :: Num a => a -> a
double n = n * 2

square :: Num a => a -> a
square n = n ^ 2

ifEven :: Integral p => (p -> p) -> p -> p
ifEven func n =
  if even n
    then func n
    else n

ifEvenInc :: Integral p => p -> p
ifEvenInc n = ifEven inc n

ifEvenDouble :: Integral p => p -> p
ifEvenDouble n = ifEven double n

ifEvenSquare :: Integral p => p -> p
ifEvenSquare n = ifEven square n

-- Using functions as arguments helped to clean up your code.
-- But you’ll notice you’re still repeating a programming pattern!
-- Each of these definitions is identical except for the function you’re passing to ifEven.
-- What you want is a function that builds ifEvenX functions.
-- To solve this, you can build a new function that returns functions, called genIfEven

-- Now you’re passing in a function and returning a lambda function.
-- The function f that you passed in is captured inside the lambda function!
-- When you capture a value inside a lambda function, this is referred to as a CLOSURE.

genIfEven :: Integral p => (p -> p) -> p -> p
genIfEven f = (\x -> ifEven f x)

ifEvenInc' :: Integer -> Integer
ifEvenInc' = genIfEven inc

ifEvenDouble' :: Integer -> Integer
ifEvenDouble' = genIfEven double

ifEvenSquare' :: Integer -> Integer
ifEvenSquare' = genIfEven square

getRequestURL :: [Char] -> [Char] -> [Char] -> [Char] -> [Char]
getRequestURL host apiKey resource id =
  host
    ++ "/"
    ++ resource
    ++ "/"
    ++ id
    ++ "?token="
    ++ apiKey

-- GHCi> getRequestURL "http://example.com" "1337hAsk3ll" "book" "1234"

-- "http://example.com/book/1234?token=1337hAsk3ll"

-- Nearly every programmer on the team will be focusing on data from just a few hosts.
-- It seems silly, not to mention error-prone, to have programmers manually type in http://example.com every time they need to make a request.
-- What you need is a function that everyone can use to generate a request URL builder just for them.

genHostRequestBuilder :: [Char] -> [Char] -> [Char] -> [Char] -> [Char]
genHostRequestBuilder host =
  ( \apiKey resource id ->
      getRequestURL host apiKey resource id --you're capturing the host argument in this lambda function
  )

googleUrlBuilder :: [Char] -> [Char] -> [Char] -> [Char]
googleUrlBuilder = genHostRequestBuilder "https://google.com"

-- GHCi> googleUrlBuilder "1337hAsk3ll" "book" "1234"

-- It’s clear you run into the same problem again when you look at apiKey.
-- Passing your API key in each time you call exampleUrlBuilder is still tedious because you’ll likely be using only one or two API keys

genApiRequestBuilder :: (t1 -> t2 -> t3 -> t4) -> t1 -> t2 -> t3 -> t4
genApiRequestBuilder hostBuilder apiKey =
  ( \resource id ->
      hostBuilder apiKey resource id
  )

myGoogleUrlBuilder :: [Char] -> [Char] -> [Char]
myGoogleUrlBuilder = genApiRequestBuilder googleUrlBuilder "1337hAsk3ll"

-- GHCi> myGoogleUrlBuilder "book" "1234"
-- "http://google.com/book/1234?token=1337hAsk3ll"
-- "http://google.com/book/1234?token=1337hAsk3ll"