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

code:hs

describeList' [] = "The list is empty."

describeList' _ = "The list has multiple elements."

データの構造を見て分岐する時に使う

code:hs

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

if式を使ったもの

code:hs

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

code:hs

describeList xs = case xs of

[] -> "The list is empty."

_ -> "The list has multiple elements."

定義