Daniel Trstenjak wrote:
Why is it not possible to combine them with a logical OR, instead of the comma that stands for a logical AND?
The '|' already acts as the "OR", why should you need another one?
Thanks for your answer. Consider the following example: ------------------ type Foo = Maybe Int f :: Foo -> Foo -> Int f x y | Just n <- x = 1 | Just n <- y = 1 | otherwise = 2 main = do print $ f (Just 3) Nothing print $ f Nothing (Just 3) print $ f (Just 3) (Just 3) print $ f Nothing Nothing ------------------ It works correctly, but I am compelled to duplicate `1` even if I know that the result of `f x y` will be `1` if either `x` or `y` is `Just n`. Here the duplication is limited in terms of number of characters, but this may not always be so. So I would like to do: ------------------ type Foo = Maybe Int f :: Foo -> Foo -> Int f x y | (Just n <- x) || (Just n <- y) = 1 | otherwise = 2 main = do print $ f (Just 3) Nothing print $ f Nothing (Just 3) print $ f (Just 3) (Just 3) print $ f Nothing Nothing ------------------ But the `||` yields a parse error, `||` is not supported in pattern guards. Whereas it is supported in classical guards: ------------------ f :: Int -> Int f n | n == 1 = 1 | n < 1 || n > 4 = 2 | otherwise = 3 main = do print $ f 1 print $ f 5 print $ f (-1) print $ f 2 ------------------ What am I missing? Thanks in advance, TP