Hey folks!

Yesterday we talked about Pattern Matching and today we will start with a special case of Pattern Matching. If we need get the whole list during a Pattern Matching we can use something called as-pattern. I don’t think this special case not much useful, but I’ll show you one example:

firstLetter :: String -> String
firstLetter all@(x:xs) = "The first letter of " ++ all ++ " is " ++ [x]


firstLetter "Haskell"
"The first letter of Haskell is H"

Now we will talk about something important. Guards are another way to do Pattern Matching where we can compare values. It is more similar with IF/ELSE than Pattern Matching is. When we receive a value we can valid if some property from it is true or false what we can’t do it with Pattern Matching, but we can with Guards. Let’s see an example:

bmiStatus :: Double -> Double -> String
bmiStatus weight height
  | weight / height ^ 2 <= 18.5 = "You are underweight!"
  | weight / height ^ 2 <= 25 = "You are with ideal weight!"
  | weight / height ^ 2 <= 30 = "You are overweight!"
  | otherwise = "You are obese!"


bmiStatus 85 1.81
"You are overweight!"


bmiStatus 80 1.81
"You are with ideal weight!"


bmiStatus 60 1.81
"You are underweight!"


bmiStatus 100 1.81
"You are obese!"

The Guards looks like Pattern Matching, but we can compare values and do actions if the comparison is true or false. Instead of Pattern Matching which evaluates the pattern.

But in this function where we are calculating if some person is fit or not comparing his/her weight and height we are repeating three times the same calculation. We can improve that using where.

bmiStatus' :: Double -> Double -> String
bmiStatus' weight height
  | bmi <= 18.5 = "You are underweight!"
  | bmi <= 25 = "You are with ideal weight!"
  | bmi <= 30 = "You are overweight!"
  | otherwise = "You are obese!"
  where bmi = weight / height ^ 2


bmiStatus’ 85 1.81
"You are overweight!"


bmiStatus’ 80 1.81
"You are with ideal weight!"


bmiStatus’ 60 1.81
"You are underweight!"


bmiStatus’ 100 1.81
"You are obese!"

Now we have our function simpler and faster because the calculation is done once. But we can improve it and make our function more readable and meaningful too.

bmiStatus' :: Double -> Double -> String
bmiStatus' weight height
  | bmi <= skinny = "You are underweight!"
  | bmi <= normal = "You are with ideal weight!"
  | bmi <= fat = "You are overweight!"
  | otherwise = "You are obese!"
  where bmi = weight / height ^ 2
           (skinny, normal, fat) = (18.5, 25, 30)

One thing which we have to be aware and take care is that where has scope for function body. Then if the same function has two bodies and the last has one where the “variables” are just available in the second body and not in all function. The where also can be used with Pattern Matching when we want save values in “variables”.

Today we learnt more two good features which can help us to write code in Haskell.