[Haskell-cafe] pattern matching on date type

[Bulat Ziganshin]

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


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> thanks!
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> suppose we have 
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>> data Tree a = Leaf a | Branch (Tree a) (Tree a)  deriving Show
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> and how I could define a function foo :: a -> Tree a that
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> foo a = Leaf  a         where a is not a type of  Tree
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> foo b =  b                 where  b is one of the type of Tree (Leaf or Branch) ?
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> The following code seems not working......
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> foo (Leaf a) = a
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> foo a = Leaf a
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> saying 'Couldn't match expected type `a' against  inferred type `Btree a''
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> any idea?
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> Thanks,
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> Max
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> From: Brandon S. Allbery KF8NH 
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> Sent: Thursday, January 01, 2009 7:35 AM
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> To: Max.cs 
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> Cc: Brandon S. Allbery KF8NH ; beginners at haskell.org ; haskell-cafe at haskell.org
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> Subject: Re: [Haskell-cafe] definition of data
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> On 2009 Jan 1, at 2:32, Max.cs wrote: 
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> data Tree a = a | Branch (Tree a) (Tree a)    deriving Show
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> but it seems not accpetable in haskell ?
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> You need a constructor in both legs of the type:
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>> data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show
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-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com