The main function `main' is not exported by module `Main'

「Real World Haskell」読書メモ 4.ライブラリを書く 5.5Haskellプログラムの生成、およびモジュールのインポート

-- file: ch05/SimpleJSON.hs

module SimpleJSON
    (
      JValue(..)
    , getString
    , getInt
    , getDouble
    , getBool
    , getObject
    , getArray
    , isNull
    ) where

data JValue = JString String
            | JNumber Double
            | JBool Bool
            | JNull
            | JObject [(String, JValue)]
            | JArray [JValue]
              deriving (Eq, Ord, Show)

getString :: JValue -> Maybe String
getString (JString s) = Just s
getString _           = Nothing

getInt :: JValue -> Maybe Int
getInt (JNumber n) = Just (truncate n)
getInt _           = Nothing
 
getDouble :: JValue -> Maybe Double
getDouble (JNumber n) = Just n
getDouble _           = Nothing
 
getBool :: JValue -> Maybe Bool
getBool (JBool b) = Just b
getBool _         = Nothing
 
getObject :: JValue -> Maybe [(String, JValue)]
getObject (JObject o) = Just o
getObject _           = Nothing
 
getArray :: JValue -> Maybe [JValue]
getArray (JArray a) = Just a
getArray _          = Nothing
 
isNull :: JValue -> Bool
isNull v            = v == JNull

-- file: ch05/Main.hs
module Main () where
 
import SimpleJSON
 
main = print (JObject [("foo", JNumber 1), ("bar", JBool False)])

PS D:\haskell\json> ghc --make Main.hs  -o json
[1 of 2] Compiling SimpleJSON       ( SimpleJSON.hs, SimpleJSON.o )
[2 of 2] Compiling Main             ( Main.hs, Main.o )

Main.hs:1:0:
    The main function `main' is not exported by module `Main'

"module Main where" に修正して全てをエクスポートするか、 "module Main (main) where" として main を指定してエクスポートすれば OK。