[Haskell-cafe] Question concerning datatype "Either"
Bayley, Alistair
Alistair.Bayley at invesco.com
Fri Jul 4 05:22:14 EDT 2008
> From: haskell-cafe-bounces at haskell.org
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of phy51km4n
>
> Why does that not work? :
>
> fromTuple (One a) = a
> fromTuple (Two a b) = (a, b)
> fromTuple (Three a b c) = (a, b, c)
> fromTuple (Four a b c d) = (a, b, c, d)
Have you tried it? What are the types of the various cases in fromTuple
(you can rename them to get ghci to accept them as separate functions):
fromTuple1 :: Tuple a b c d -> a
fromTuple1 (One a) = a
fromTuple2 :: Tuple a b c d -> ???
fromTuple2 (Two a b) = (a, b)
> Why is this correct? :
>
> fromTuple (One a ) = Left (Left a)
> fromTuple (Two a b ) = Left (Right (a,b))
> fromTuple (Three a b c ) = Right (Left (a,b,c))
> fromTuple (Four a b c d) = Right (Right (a,b,c,d))
Again, what is the type of just this line?
fromTuple (One a ) = Left (Left a)
and this line?
fromTuple (Two a b ) = Left (Right (a,b))
And if you combine them?
fromTuple :: Tuple a b c d -> ???
fromTuple (One a ) = Left (Left a)
fromTuple (Two a b ) = Left (Right (a,b))
(You should see that Either's type parameters are progressively
filled-in as you add more cases.)
Alistair
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