where

where を使って計算の中間結果に名前をつけることが出来る。
ただしwhere にはスコープがあり、その関数の中でしか見えないため、注意すること。

bmiTell'' :: Double -> Double -> String
bmiTell'' weight height
  | bmi <= 18.5 = "You're underweight"
  | bmi  <= 25.0 = "You're normal"
  | bmi  <= 30.0 = "You're fat"
  | otherwise = "You're a whale"
  where bmi = weight / height ^ 2

bmiTell''' :: Double -> Double -> String
bmiTell''' weight height
  | bmi <= 18.5 = "You're underweight"
  | bmi  <= 25.0 = "You're normal"
  | bmi  <= 30.0 = "You're fat"
  | otherwise = "You're a whale"
  where bmi = weight / height ^ 2
        skinny = 18.5
        normal = 25.0
        fat = 30.0

--関数の外からwhere を使おうとしているため、エラーが返る
-- greet :: String -> String
-- greet "Juan" = niceGreeting ++ " Juan"
-- greet "Fernando" = niceGreeting ++ "Fernando"
-- greet name = badGreeting ++ " " ++ name
--   where niceGreeting = "Hello, "
--         badGreeting = "Oh! Pfft."

badGreeting :: String
badGreeting = "Pfft "
niceGreeting :: String
niceGreeting = "Hello, "
greet :: String -> String
greet "Juan" = niceGreeting ++ " Juan"
greet "Fernando" = niceGreeting ++ " Fernando"
greet name = badGreeting ++ " " ++ name

initials :: String -> String -> String
initials firstname lastname = [f] ++ ". " ++ [l] ++ "."
  where (f:_) = firstname
        (l:_) = lastname

calcBmis :: [(Double, Double)] -> [Double]
calcBmis xs = [bmi w h | (w, h) <- xs]
  where bmi weight height = weight / height ^ 2

結果

Main> bmiTell'' 85 1.90
"You're normal"
Main> bmiTell''' 85 1.90
"You're normal"
Main> greet "Juan"
"Hello,  Juan"
Main> greet "Fernando"
"Hello,  Fernando"
Main> let name = "Hoge"
Main> greet name
"Pfft  Hoge"
Main> initials "Michael" "Kheel"
"M. K."
Main> calcBmis [(85.0,1.90)]
[23.545706371191137]