The (x:xs) in function parameter is a tuple?

[Nan Xiao]

Hi all,

Greetings from me!

I am confused about the function parameters and tuple. E.g.:

occurs value [] = 0
occurs value (x:xs) = (if value == x then 1 else 0) + occurs value xs

should we consider (x:xs) as a tuple?

Thanks in advance!

Best Regards
Nan Xiao
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[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

(:) is a constructor. For example, you can define lists as:

data List a = Nil | Cons a (List a)

GHC does some magic to provide us with the same definition, but with Nil replaced by [] and Cons replaced by (:).

As constructors can be pattern matched on, you can also match on a (:), which is a data constructor.

You might consider (x:xs) as a tuple, only if you're willing to consider (Cons x xs) as a tuple. It is a tuple (ordered collection of two values), but not a tuple according to their definition in haskell.

What kind of tuple are you talking about?

Hope this helps.

On 24 February 2016 at 16:01, Nan Xiao <[hidden email]> wrote:

Hi all,

Greetings from me!

I am confused about the function parameters and tuple. E.g.:

occurs value [] = 0
occurs value (x:xs) = (if value == x then 1 else 0) + occurs value xs

should we consider (x:xs) as a tuple?

Thanks in advance!

Best Regards
Nan Xiao
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[Imants Cekusins]

Hello Xiao,

(one_element)             -- is evaluation
(element,element,...)    -- is tuple

(1:[2])     -- [1,2] because it is one "array element"
(1,2)      -- is a tuple because there are 2 elements
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[Rein Henrichs]

A tuple can have any number of elements. (1,2) is a 2-tuple. (1,2,3) is a 3-tuple. (x:xs) and (x,y) are differently typed: xs must be a list of the type of x, while y can be any type. (x:xs) is not a tuple. It is a list by definition.

On Wed, Feb 24, 2016 at 2:45 AM Imants Cekusins <[hidden email]> wrote:

Hello Xiao,

(one_element)             -- is evaluation
(element,element,...)    -- is tuple

(1:[2])     -- [1,2] because it is one "array element"
(1,2)      -- is a tuple because there are 2 elements
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[Graham Gill]

Hi Nan, are you just confused about the use of the parentheses "(" and ")"?

(x1,x2), (x1,x2,x3), ... are tuples in Haskell, but (x:xs) is not.
(There's no "one-ple", or 1-tuple, in Haskell.) In

occurs value [] = 0
occurs value (x:xs) = (if value == x then 1 else 0) + occurs value xs

the "(" and ")" around "x:xs" are just there for grouping, for operator
precedence reasons. Function application binds more tightly than ":". If
you leave the parentheses off, such as in

occurs value x:xs = ...

you'll get a parse error.

Graham


On 2/24/2016 5:31 AM, Nan Xiao wrote:

> Hi all,
>
> Greetings from me!
>
> I am confused about the function parameters and tuple. E.g.:
>
> occurs value [] = 0
> occurs value (x:xs) = (if value == x then 1 else 0) + occurs value xs
>
> should we consider (x:xs) as a tuple?
>
> Thanks in advance!
>
> Best Regards
> Nan Xiao
> _______________________________________________
> Beginners mailing list
> [hidden email]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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[Nan Xiao]

Hi all,

Firstly, thanks very much for all responses!

Rein referred "A tuple can have any number of elements", while Graham
referred "There's no "one-ple", or 1-tuple, in Haskell.". So which one
is right? The tuple at least contains 2 elements?

Thanks very much in advance!
Best Regards
Nan Xiao


On Thu, Feb 25, 2016 at 2:37 AM, Graham Gill <[hidden email]> wrote:

> Hi Nan, are you just confused about the use of the parentheses "(" and ")"?
>
> (x1,x2), (x1,x2,x3), ... are tuples in Haskell, but (x:xs) is not. (There's
> no "one-ple", or 1-tuple, in Haskell.) In
>
> occurs value [] = 0
> occurs value (x:xs) = (if value == x then 1 else 0) + occurs value xs
>
> the "(" and ")" around "x:xs" are just there for grouping, for operator
> precedence reasons. Function application binds more tightly than ":". If you
> leave the parentheses off, such as in
>
> occurs value x:xs = ...
>
> you'll get a parse error.
>
> Graham
>
>
>
> On 2/24/2016 5:31 AM, Nan Xiao wrote:
>>
>> Hi all,
>>
>> Greetings from me!
>>
>> I am confused about the function parameters and tuple. E.g.:
>>
>> occurs value [] = 0
>> occurs value (x:xs) = (if value == x then 1 else 0) + occurs value xs
>>
>> should we consider (x:xs) as a tuple?
>>
>> Thanks in advance!
>>
>> Best Regards
>> Nan Xiao
>> _______________________________________________
>> Beginners mailing list
>> [hidden email]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> _______________________________________________
> Beginners mailing list
> [hidden email]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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[Imants Cekusins]

My guess is: tuple must contain 2+ elements.

Try to enter (1) in ghci. It is displayed as 1

Parentheses are only recognized as a tuple if there are elements separated by a comma. Otherwise an expression is assumed.

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[Josh Barney]

The way I learned it:

A tuple type is defined by the types inside it, so an (int,int) is not the same type as an (int, maybe(int)). With a one-tuple the type would just be (int) and that provides zero benefit. 

Instead of trying to do `first (...my expression...)` to get the first element of the one tuple, it just becomes the value inside.

Sent from my iPhone

On Feb 24, 2016, at 7:02 PM, Imants Cekusins <[hidden email]> wrote:

My guess is: tuple must contain 2+ elements.

Try to enter (1) in ghci. It is displayed as 1

Parentheses are only recognized as a tuple if there are elements separated by a comma. Otherwise an expression is assumed.

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[Max Voit-2]

On Thu, 25 Feb 2016 07:49:56 +0800
Nan Xiao <[hidden email]> wrote:

> Rein referred "A tuple can have any number of elements", while Graham
> referred "There's no "one-ple", or 1-tuple, in Haskell.". So which one
> is right? The tuple at least contains 2 elements?

There is no one-tuple; however, there's a zero-tuple. Also no arbitrary
number of tuple elements allowed, due to definition (we're at 61 for
some reason). Refer to

http://hackage.haskell.org/package/ghc-prim-0.4.0.0/docs/GHC-Tuple.html

best, Max
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[Rein Henrichs]

Nan, generally speaking (e.g., in maths) tuples can contain any number of elements. Haskell specifically disallows one-tuples because of the ambiguity in parsing them. Instead, (expr) is just evaluated as expr. On the other hand, it does allow (), which is a zero-tuple. Haskell's compilers also impose an upper bound on the practical size of a tuple (for modern GHC, that's 61, as Max mentioned) but that isn't a restriction of the Haskell language itself. If you want, you can consider Identity a to be a one-tuple, but it obviously doesn't use tuple syntax.

As a side note, a 1-tuple is occasionally called a monad in other disciplines, while a monad is actually defined as a triple (a three-tuple) in maths. None of that is relevant to Haskell though.

On Thu, Feb 25, 2016 at 7:14 AM Max Voit <[hidden email]> wrote:

On Thu, 25 Feb 2016 07:49:56 +0800
Nan Xiao <[hidden email]> wrote:

> Rein referred "A tuple can have any number of elements", while Graham
> referred "There's no "one-ple", or 1-tuple, in Haskell.". So which one
> is right? The tuple at least contains 2 elements?

There is no one-tuple; however, there's a zero-tuple. Also no arbitrary
number of tuple elements allowed, due to definition (we're at 61 for
some reason). Refer to

http://hackage.haskell.org/package/ghc-prim-0.4.0.0/docs/GHC-Tuple.html

best, Max
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