在Haskell中如何解析撇号/字符文字?

我定义了Lambda表达式的类型:

data Expression = Variable String
                | Lambda Expression Expression 
                | Apply Expression Expression 

processInput :: String -> Expression
processInput ('\':input) = processLambda input
processInput ('(':input) = processApply input
processInput str         = Variable str

当我尝试加载此函数时,我得到

lexical error in string/character literal at ':'

所以我试着用警卫代替:

processInput input
    | head input == '\'                      = processLambda (tail input)
    | head input == '('                      = processApply (tail input)
    | otherwise                              = Variable input

lexical error in string/character literal at character ' '