HaskellのPattern Matching
内部的にはcase式のみ
以下に示すものは全てcase式の糖衣構文
関数を複数回行うもの
hsdescribeList' :: [a] -> String
describeList' [] = "The list is empty."
describeList' [x] = "The list has one element."
describeList' _ = "The list has multiple elements."データの構造を見て分岐する時に使う
guardを使ったもの
hsdescribeListWithGuard :: [a] -> String
describeListWithGuard xs
| length xs == 0 = "The list is empty."
| length xs == 1 = "The list has one element."
| otherwise = "The list has multiple elements."データの性質で分岐する時に使う
otherwise で網羅性をチェックするif式を使ったもの
hsisEmptyIf :: [a] -> Bool
isEmptyIf xs = if null xs then True else False条件分岐が少数の場合に使う
case..ofを使ったもの
hsdescribeList :: [a] -> String
describeList xs = case xs of
[] -> "The list is empty."
[x] -> "The list has one element."
_ -> "The list has multiple elements."これだけ
-> を使うのが紛らわしい定義