リストと値の型

ひとつのリスト中に違う型の値を含めることはできない。

Prelude> [1,2,'a']
<interactive>:1:1:
    No instance for (Num Char)
      arising from the literal 1' at <interactive>:1:1
    Probable fix: add an instance declaration for (Num Char)
    In the list element: 1
    In the definition of it': it = [1, 2, 'a']

当然連結もダメ。

Prelude> [1,2,3] ++ ['a','b','c']
<interactive>:1:1:
    No instance for (Num Char)
      arising from the literal 1' at <interactive>:1:1
    Probable fix: add an instance declaration for (Num Char)
    In the list element: 1
    In the first argument of (++)', namely [1, 2, 3]'
    In the definition of it': it = [1, 2, 3] ++ ['a', 'b', 'c']