Re: [Haskell-beginners] ignature for a list of a set length

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

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