=====================
Lazy Pattern Matching
=====================

看代码，不解释。

普通pattern match：

    .. code-block:: haskell

        Prelude> let f (x, y) = [1, x]
        Prelude> let a = f undefined
        Prelude> head a
        *** Exception: Prelude.undefined

Lazy pattern matching：

    .. code-block:: haskell

        Prelude> let f ~(x, y) = [1, x]
        Prelude> let a = f undefined
        Prelude> head a
        1
        Prelude> a !! 1
        *** Exception: Prelude.undefined