[Haskell-cafe] pattern matching on date type
thanks! suppose we have data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show and how I could define a function foo :: a - Tree a that foo a = Leaf a where a is not a type of Tree foo b = b where b is one of the type of Tree (Leaf or Branch) ? The following code seems not working.. foo (Leaf a) = a foo a = Leaf a saying 'Couldn't match expected type `a' against inferred type `Btree a'' any idea? Thanks, Max From: Brandon S. Allbery KF8NH Sent: Thursday, January 01, 2009 7:35 AM To: Max.cs Cc: Brandon S. Allbery KF8NH ; beginn...@haskell.org ; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] definition of data On 2009 Jan 1, at 2:32, Max.cs wrote: data Tree a = a | Branch (Tree a) (Tree a) deriving Show but it seems not accpetable in haskell ? You need a constructor in both legs of the type: data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] pattern matching on date type
Hello Max.cs, Thursday, January 1, 2009, 11:36:24 AM, you wrote: seems that you come from dynamic languages :) Haskell has static typing meaning that your function can accept either Tree or a as arguments. so you should convert a to Tree explicitly, using Leaf thanks! suppose we have data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show and how I could define a function foo :: a - Tree a that foo a = Leaf a where a is not a type of Tree foo b = b where b is one of the type of Tree (Leaf or Branch) ? The following code seems not working.. foo (Leaf a) = a foo a = Leaf a saying 'Couldn't match expected type `a' against inferred type `Btree a'' any idea? Thanks, Max From: Brandon S. Allbery KF8NH Sent: Thursday, January 01, 2009 7:35 AM To: Max.cs Cc: Brandon S. Allbery KF8NH ; beginn...@haskell.org ; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] definition of data On 2009 Jan 1, at 2:32, Max.cs wrote: data Tree a = a | Branch (Tree a) (Tree a)deriving Show but it seems not accpetable in haskell ? You need a constructor in both legs of the type: data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
