On 05.06.2011 14:40, Haisheng Wu wrote:
By looking at sequence implementation in Hugs:
sequence (c:cs) = do x<- c xs<- sequence cs return (x:xs)
Apply it against a list [Just 2, Nothing], we will have: sequence [Just 2, Nothing] = do x<- Just 2 y<- sequence [Nothing] return (x:y) = return (2:Nothing)
The question is Why/How `return (2:Nothing)` eval to `Nothing` ?
It doesn't. You went wrong in the last equality there. y doesn't have the value Nothing, in fact it never gets a value at all. Let's first look at what sequence [Nothing] evaluates to: do x <- Nothing y <- sequence [Nothing] return (x:y) Since Nothing >>= f evaluates to Nothing this means that the above do-block evaluates to Nothing, too. So y <- sequence [Nothing] becomes y <- Nothing and again the whole expression evaluates to Nothing and return (x:y) never is evaluated.