Real World Haskell Chapter3

Nullable.hsはNulllabel.hsのtypo?

--Nullable.hs
data Maybe a = Just a
             | Nothing

someBool = Just True

someString = Just "something"

wrapped = Just (Just "wrapped")

写経すると以下のエラー.
よく分からんが，定義が重複しているみたい？

[1 of 1] Compiling Main             ( Nullable.hs, interpreted )

Nullable.hs:5:11:
    Ambiguous occurrence `Just'
    It could refer to either `Main.Just', defined at Nullable.hs:2:15
                          or `Prelude.Just', imported from Prelude

--Nullable.hs
data Maybe a = Just a
             | Nothing

someBool = Main.Just True

someString = Main.Just "something"

wrapped = Main.Just (Main.Just "wrapped")

Scopeを制限してみようと，以上に改造してみた．
読み込みはできるけど，以下のエラーが発生する．
MaybeとかJustとかよく分からん．

[1 of 1] Compiling Main             ( Nullable.hs, interpreted )
Ok, modules loaded: Main.
*Main> some
someBool    someString
*Main> someBool

<interactive>:1:0:
    No instance for (Show (Main.Maybe Bool))
      arising from a use of `print' at <interactive>:1:0-7
    Possible fix:
      add an instance declaration for (Show (Main.Maybe Bool))
    In a stmt of a 'do' expression: print it

分からないところは飛ばして練習問題．

--ListADT.hs
data List a = Cons a (List a)
            | Nil 
              deriving (Show)

fromList :: [t] -> List t
fromList (x:xs) = Cons x (fromList xs) 
fromList [] = Nil 

toList :: List t -> [t] 
toList Nil = []
toList xs = [headList xs] ++ toList (tailList xs) 

headList (Cons x _) = x 
tailList (Cons _ xs) = xs

2問目はMaybeを使えと書いてあるが．．．
ここで挫折の予感がします．

3.9.2 Where節

lend関数とlend2関数の動作が全然違う．
ということで，同じ動作の関数をlend3として作ってみた．

--Lending.hs
lend amount balance = let reserve = 100 
                          newBalance = balance - amount
                      in if balance < reserve
                         then Nothing
                         else Just newBalance

lend2 amount balance    = if amount < reserve * 0.5 
                          then Just newBalance
                          else Nothing
      where reserve     = 100 
            newBalance  = balance - amount

lend3 amount balance    = if balance < reserve
                          then Nothing
                          else Just newBalance
      where reserve     = 100 
            newBalance  = balance - amount

しかも，lend2は引数によってはsegv起こすし．．．
初心者の私には原因がわかりません．

[1 of 1] Compiling Main             ( Lending.hs, interpreted )
Ok, modules loaded: Main.
*Main> lend2 15 10
Just (-[1]    6301 segmentation fault  ghci