[Haskell-cafe] Missing a "Deriving"?

michael rice nowgate at yahoo.com
Sun May 31 00:55:45 EDT 2009

Belay that last question. I just realized that its the const function being used rather than a constant declaration in const Nothing and const [].


--- On Sat, 5/30/09, David Menendez <dave at zednenem.com> wrote:

From: David Menendez <dave at zednenem.com>
Subject: Re: [Haskell-cafe] Missing a "Deriving"?
To: "michael rice" <nowgate at yahoo.com>
Cc: "Miguel Mitrofanov" <miguelimo38 at yandex.ru>, haskell-cafe at haskell.org
Date: Saturday, May 30, 2009, 9:33 PM

On Sat, May 30, 2009 at 9:00 PM, michael rice <nowgate at yahoo.com> wrote:
> That works. but it gives just a single solution [1,2,3] when there are
> supposed to be two [[1,2,3],[1,4,3]]. Of course the code in YAHT may be in
> error.

Works for me.

*Main> searchAll g 1 3 :: [[Int]]
*Main> searchAll g 1 3 :: Maybe [Int]
Just [1,2,3]
*Main> searchAll g 1 3 :: Failable [Int]
Success [1,2,3]

> Also, how the heck does Haskell decide which "success", "failure",
> "augment", and "combine" to use in function "searchAll", since there are
> five possibilities.

*Main> :t searchAll
searchAll :: (Computation c) => Graph t t1 -> Int -> Int -> c [Int]

The way searchAll is written, the choice of which functions to use
depends on the type variable c. That's determined by the calling
context of searchAll, which is why you need to provide a type
signature when using it at the GHCi command line.

Dave Menendez <dave at zednenem.com>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090531/b582fa79/attachment.html

More information about the Haskell-Cafe mailing list