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Martin Tomko

28 Sep 2010 28 Sep '10

10:07 a.m.

Dear all, I want to have a function that acts only on lists of length 3 (I have a function that filters a list of lists and returns only those of that length). I guess I could change them into tuples (is there a way?), but anyway. Is there a way to specify in the signature that the function should only match such lists, or do I have to do pattern mattching in order to exclude other possibilities?: getTernaryRelationship :: [a] -> [a] getTernaryRelationship ls = if (length ls /= 3) then error "not a ternary rel" else ... thanks Martin

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Ozgur Akgun

28 Sep 28 Sep

10:41 a.m.

Hi, On 28 September 2010 15:07, Martin Tomko wrote:

...

I want to have a function that acts only on lists of length 3 (I have a function that filters a list of lists and returns only those of that length). I guess I could change them into tuples (is there a way?)

Well if both the producer and the consumer of this value are your own functions, you can use something other than a list. You can also implement a length indexed list, or use such an implementation but it will be overkill for this task, I think. The two options are either implementing a wrapper type with three fields, or using a 3-tuple. I would use a 3-tuple in both the producer and consumer functions. Best, Ozgur

edgar klerks

11:04 a.m.

And if that isn't a possibility, you can use pattern matching: bla (x:y:z:[]) = do something bla _ = do nothing On Tue, Sep 28, 2010 at 4:41 PM, Ozgur Akgun wrote:

...

Hi,

On 28 September 2010 15:07, Martin Tomko wrote:

...
I want to have a function that acts only on lists of length 3 (I have a function that filters a list of lists and returns only those of that length). I guess I could change them into tuples (is there a way?)

Well if both the producer and the consumer of this value are your own functions, you can use something other than a list. You can also implement a length indexed list, or use such an implementation but it will be overkill for this task, I think.

The two options are either implementing a wrapper type with three fields, or using a 3-tuple. I would use a 3-tuple in both the producer and consumer functions.

Best, Ozgur

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Jonas Almström Duregård

11:05 a.m.

A slightly more readable (IMO) version (which won't fail to terminate on infinite lists): getTernaryRelationship (ls@[a,b,c]) = ... getTernaryRelationship _ = error "getTernaryRelationship: not a ternary rel" You can of course have type (a,a,a) -> (a,a,a) unless your other functions require lists /J On 28 September 2010 16:07, Martin Tomko wrote:

...

Dear all, I want to have a function that acts only on lists of length 3 (I have a function that filters a list of lists and returns only those of that length). I guess I could change them into tuples (is there a way?), but anyway. Is there a way to specify in the signature that the function should only match such lists, or do I have to do pattern mattching in order to exclude other possibilities?:

getTernaryRelationship :: [a] -> [a] getTernaryRelationship ls = if (length ls /= 3) then error "not a ternary rel" else ...

thanks Martin

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edgar klerks

11:20 a.m.

I think it is simpler to use the Maybe type for this. If you only search for ternary relations, you can write a function, which only converts a list to a 3-tuple if it is of length three and otherwise it returns nothing. Then with catMaybes :: [Maybe a] -> [a], you can convert it back to a list with only 3-tuples. import Data.Maybe getLengthThree = catMaybes . fmap toTup where toTup (x:y:z:[]) = Just (x,y,z) toTup _ = Nothing

Martin Tomko

11:39 a.m.

I like this suggestion, I will give it a go! thanks, and also for the catMaybes function, I needed something like that some days ago. martin On 9/28/2010 5:20 PM, edgar klerks wrote:

...

I think it is simpler to use the Maybe type for this. If you only search for ternary relations, you can write a function, which only converts a list to a 3-tuple if it is of length three and otherwise it returns nothing. Then with catMaybes :: [Maybe a] -> [a], you can convert it back to a list with only 3-tuples.

import Data.Maybe

getLengthThree = catMaybes . fmap toTup where toTup (x:y:z:[]) = Just (x,y,z) toTup _ = Nothing

-- Martin Tomko Postdoctoral Research Assistant Geographic Information Systems Division Department of Geography University of Zurich - Irchel Winterthurerstr. 190 CH-8057 Zurich, Switzerland email: martin.tomko@geo.uzh.ch site: http://www.geo.uzh.ch/~mtomko mob: +41-788 629 558 tel: +41-44-6355256 fax: +41-44-6356848

Martin Tomko

11:41 a.m.

David Virebayre

29 Sep 29 Sep

3:23 a.m.

2010/9/28 Martin Tomko :

...

As responded earlier, I may try the maybes, but I am intrigued by the ls@[a,b,c] and I cannto find any documentation about it. If you would not

When pattern matching a list of 3 elements, - ls is the whole 3-element list - a is the first element - b is the second element - c is the last. You can use it anytime you need to deconstruct a value, but also have a name for the value. Examples m@Just j l@(x:xs) ...

...

mind, could you enlighten me? Thanks Martin

On 9/28/2010 5:05 PM, Jonas Almström Duregård wrote:

...
A slightly more readable (IMO) version (which won't fail to terminate on infinite lists):

getTernaryRelationship (ls@[a,b,c]) = ... getTernaryRelationship _ = error "getTernaryRelationship: not a ternary rel"

You can of course have type (a,a,a) -> (a,a,a) unless your other functions require lists

/J

On 28 September 2010 16:07, Martin Tomko wrote:

...
Dear all, I want to have a function that acts only on lists of length 3 (I have a function that filters a list of lists and returns only those of that length). I guess I could change them into tuples (is there a way?), but anyway. Is there a way to specify in the signature that the function should only match such lists, or do I have to do pattern mattching in order to exclude other possibilities?:

getTernaryRelationship :: [a] -> [a] getTernaryRelationship ls = if (length ls /= 3) then error "not a ternary rel" else ...

thanks Martin

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- Martin Tomko Postdoctoral Research Assistant

Geographic Information Systems Division Department of Geography University of Zurich - Irchel Winterthurerstr. 190 CH-8057 Zurich, Switzerland

email: martin.tomko@geo.uzh.ch site: http://www.geo.uzh.ch/~mtomko mob: +41-788 629 558 tel: +41-44-6355256 fax: +41-44-6356848

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participants (5)

David Virebayre
edgar klerks
Jonas Almström Duregård
Martin Tomko
Ozgur Akgun