mapM (const ['0'..'9']) [1..3] がよく分からない
展開してみれば分かるはず。
mapM (const ['0'..'9']) [1..3] -- -> ['000', '001', '002', ..., '998', '999] sequence [['0'..'9'], ['0'..'9'], ['0'..'9']] do { x <- ['0'..'9']; xs <- do { x' <- ['0'..'9']; xs' <- do { x'' <- ['0'..'9']; xs'' <- return []; return (x'':xs'') }; return (x':xs') }; return (x:xs) } {- do { x <- ['0'..'9']; xs <- do { x' <- ['0'..'9']; xs' <- do { x'' <- ['0'..'9']; xs'' <- return []; return (x'':xs'') }; return (x':xs') }; return (x:xs) } -} ['0'..'9'] >>= \x -> (['0'..'9'] >>= \x' -> (['0'..'9'] >>= \x'' -> [[]] >>= \xs'' -> [x'':xs'']) >>= \xs' -> [x':xs']) >>= \xs -> [x:xs] {- ['0'..'9'] >>= \x -> (['0'..'9'] >>= \x' -> (concatMap (\xs'' -> concatMap (\x'' -> [x'':xs'']) ['0'..'9']) [[]]) >>= \xs' -> [x':xs']) >>= \xs -> [x:xs] -} concatMap (\xs -> concatMap (\x -> [x:xs]) ['0'..'9']) (concatMap (\xs' -> concatMap (\x' -> [x':xs']) ['0'..'9']) (concatMap (\xs'' -> concatMap (\x'' -> [x'':xs'']) ['0'..'9']) [[]]))
と期待したけど、そうでもなかった。
sequence の定義は ここ を参考に。