[英]Is it possible to have an if function use pattern matching in Haskell?
[英]Is it possible in Haskell to write the catch-all pattern first? Or use “negative” pattern?
有时,如果模式规则需要一些特殊的 rhs,通过where
使其更具可读性,我最终会得到这样的结果
data D = A | B | C
func :: D -> b
func A = special_case
where
special_case = other helper
other = aaaa
helper = bbb
func _ = bla
where 包罗万象的模式似乎与其他模式相去甚远,因为where
很长。 如果我能写出这样的东西就好了:
func :: D -> b
func !A = bla -- imaginary syntax
func A = special_case
where
special_case = other helper
other = aaaa
helper = bbb
我认为它仍然不会被称为包罗万象,而是“包罗万象”,但是有什么办法可以做到这一点吗?
如果你不需要绑定任何东西,你可以这样做:
isA :: D -> Bool
isA A = True
isA _ = False
func :: D -> b
func d | not (isA d) = bla
func A = special_case where ...
(您可以交替实现isn'tA
来避免not
。但是,虽然我经常看到像isA
这样定义的函数,但我不相信我曾经在野外见过isn'tA
的类似物。)
对于编译器来说,这个匹配是否完整可能并不明显。 你可以像这样解决这个问题:
type A'sFields = () -- your specific D doesn't have any fields for A,
-- but this pattern is general enough to use for
-- constructors that do
fromA :: D -> Maybe A'sFields
fromA A = Just ()
fromA _ = Nothing
func :: D -> b
func d = case fromA d of
Nothing -> bla
Just () -> special_case where ...
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