Thanks Senastian!
I would refine the equality as below:
sequence [Just 2, Nothing]
= do x <- Just 2
y <- sequence [Nothing]
return (x:y)
= Just 2 >>= \x -> sequence [Nothing] >>= \y -> return (x:y)
= Nothing >>= \y -> return (2:y)
= Nothing
-Haisheng
On Mon, Jun 6, 2011 at 12:31 AM, Sebastian Hungerecker
<sepp2k@googlemail.com> wrote:
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` ?
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.