Using `++` in pattern matching

Question

Typing the following into a GHC interpreter

let describe' all@([x] ++ [y]) = "The first letter of " ++ all ++ " is " ++ [x]

yields

Parser error in pattern: [x] ++ [y]

Why is Haskell unable to match the pattern all@([x] ++ [y]) against expressions like "HI" or [1,2]?

Answer 1

let's assume you could pattern-match on ++ - now think about how you could match this:

a ++ b = [1,2]

you could have:

• a = [1,2], b = []
• a = [1], b = [2]
• a = [], b = [1,2]

now what is the right one?

---

the technical reason is that ++ is not data-constructor ;)

---

in your specific situation you could use

let describe' all@[x,y] = "The first letter of " ++ all ++ " is " ++ [x]

(which will only match strings with length exactly 2)

or better

let describe' all@(x:_) = "The first letter of " ++ all ++ " is " ++ [x]

(which will match all strings of length at least 1)

a safe version would be this

describe' :: String -> String
describe' ""        = "your input was empty"
describe' all@(x:_) = "The first letter of " ++ all ++ " is " ++ [x]