引数をチェックして、問題があれば Left に包んだエラーメッセージに、問題がなければ Right に包んだ計算結果に評価される関数を考えてみよう。
純粋な関数でもロジックを表現することは可能だが、if文のネストが深くなればなるほど地獄である。手続き型言語であれば早期リターンで書きたいところだが、関数型言語ではどうするのか。
checkName :: String -> String -> Either String String
checkName fstname famname =
if ( length fstname == 0 )
then Left "error: fstname is empty"
else
if not (isValidFirstName fstname)
then Left "error: fstname is invalid"
else
if ( length famname == 0 )
then Left "error: famname is empty"
else
if not (isValidFamilyName famname)
then Left "error: famname is invalid"
else Right $ fstname ++ " " ++ famname
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)
方法1. Eitherモナドを使う
※某所で指摘を受けたので追記。
Either モナド自体の性質を利用すれば、無尽蔵にネストが深くなっていく状況は避けることができる。場合によってはこれで十分だろう。ただ、より早期リターン風に表現したいときどうしたらよいか？
checkNameEM :: String -> String -> Either String String
checkNameEM fstname famname = do
-- fstname 単体でチェック
fstname' <- if ( length fstname == 0 )
then Left "error: fstname is empty"
else
if not (isValidFirstName fstname)
then Left "error: fstname is invalid"
else Right fstname
-- famname 単体でチェック
famname' <- if ( length famname == 0 )
then Left "error: famname is empty"
else
if not (isValidFamilyName famname)
then Left "error: famname is invalid"
else Right famname
-- fstname' famname' の両方が Right の時だけ関数が評価される
return $ fstname' ++ famname'
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)
ついでにApplicativeスタイルでの記述も載せておく。
checkNameEA :: String -> String -> Either String String
checkNameEA fstname famname = do
-- fstname 単体でチェックした結果をEither(Left or Right)で包む
let fstname' = if ( length fstname == 0 )
then Left "error: fstname is empty"
else
if not (isValidFirstName fstname)
then Left "error: fstname is invalid"
else Right fstname
-- famname 単体でチェックした結果をEither(Left or Right)で包む
let famname' = if ( length famname == 0 )
then Left "error: famname is empty"
else
if not (isValidFamilyName famname)
then Left "error: famname is invalid"
else Right famname
-- Applicativeスタイル
-- fstname' famname' の両方が Right の時だけ関数が評価される
(\fst fam -> fst ++ " " ++ fam) <$> fstname' <*> famname'
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)
方法2. 継続モナドを使う
そこで継続モナド登場。純粋関数内で、早期リターンのような記述が可能となる。
checkNameC :: String -> String -> Either String String
checkNameC fstname famname = (`runCont` id) $ callCC $ \exit -> do
when ( length fstname == 0 )
$ exit $ Left "error: fstname is empty"
when ( not (isValidFirstName fstname))
$ exit $ Left "error: fstname is invalid"
when ( length famname == 0 )
$ exit $ Left "error: famname is empty"
when ( not (isValidFamilyName famname))
$ exit $ Left "error: famname is invalid"
return $ Right $ fstname ++ " " ++ famname
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)
全部のせておく。
import Data.Char(isLower,isUpper)
import Control.Monad.Cont
checkName :: String -> String -> Either String String
checkName fstname famname =
if ( length fstname == 0 )
then Left "error: fstname is empty"
else
if not (isValidFirstName fstname)
then Left "error: fstname is invalid"
else
if ( length famname == 0 )
then Left "error: famname is empty"
else
if not (isValidFamilyName famname)
then Left "error: famname is invalid"
else Right $ fstname ++ " " ++ famname
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)
checkNameEM :: String -> String -> Either String String
checkNameEM fstname famname = do
-- fstname 単体でチェック
fstname' <- if ( length fstname == 0 )
then Left "error: fstname is empty"
else
if not (isValidFirstName fstname)
then Left "error: fstname is invalid"
else Right fstname
-- famname 単体でチェック
famname' <- if ( length famname == 0 )
then Left "error: famname is empty"
else
if not (isValidFamilyName famname)
then Left "error: famname is invalid"
else Right famname
-- fstname' famname' の両方が Right の時だけ関数が評価される
return $ fstname' ++ famname'
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)
checkNameEA :: String -> String -> Either String String
checkNameEA fstname famname = do
-- fstname 単体でチェックした結果をEither(Left or Right)で包む
let fstname' = if ( length fstname == 0 )
then Left "error: fstname is empty"
else
if not (isValidFirstName fstname)
then Left "error: fstname is invalid"
else Right fstname
-- famname 単体でチェックした結果をEither(Left or Right)で包む
let famname' = if ( length famname == 0 )
then Left "error: famname is empty"
else
if not (isValidFamilyName famname)
then Left "error: famname is invalid"
else Right famname
-- Applicativeスタイル
-- fstname' famname' の両方が Right の時だけ関数が評価される
(\fst fam -> fst ++ " " ++ fam) <$> fstname' <*> famname'
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)
checkNameC :: String -> String -> Either String String
checkNameC fstname famname = (`runCont` id) $ callCC $ \exit -> do
when ( length fstname == 0 )
$ exit $ Left "error: fstname is empty"
when ( not (isValidFirstName fstname))
$ exit $ Left "error: fstname is invalid"
when ( length famname == 0 )
$ exit $ Left "error: famname is empty"
when ( not (isValidFamilyName famname))
$ exit $ Left "error: famname is invalid"
return $ Right $ fstname ++ " " ++ famname
where
isValidFirstName ca = (all isLower ca)
&& ( 4 <= length ca && length ca <= 10)
isValidFamilyName ca = (all isUpper ca)
&& ( 2 <= length ca && length ca <= 8)