I have a follow up question, I hope it's alright. What about the following? data List a = Empty | Add a (List a) What does the a mean and why is it possible to put it there? On Tue, Sep 22, 2015 at 4:54 PM, goforgit . wrote:

Thanks Brandon, that cleared things up!

On Mon, Sep 21, 2015 at 8:39 PM, Brandon Allbery wrote:

...

On Mon, Sep 21, 2015 at 2:23 PM, goforgit . wrote:

...

Could someone explain to me the difference between the following?

data Atype = Numeric Integer | A | B C

and

data Atype = Integer | A | B C

The second one is an enumeration with three values: "Integer" (which is *not* an integer, nor does it contain one), "A", and "B" which as written there takes a value of some unspecified type C as a parameter. The first one is a enumeration with three values: "Numeric" which takes an Integer as a parameter, "A", and "B" which takes a value of some unspecified type C as a parameter.

Note that the "Integer" in the second one has *nothing whatsoever* to do with the *type* Integer.

Remember that you must always provide a data constructor with "data"; you cannot simply say "data MyInt = Integer" to "wrap" an Integer, because you have not said what to wrap it *in*. (You may have intended to create a type alias, though; that would be "type", not "data".) A "data" always requires a data constructor name, so the compiler can tell when you are talking about a value of that type by looking for the constructor.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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What you wrote is equivalent to the following pseudo-haskell. *List* takes a type and returns a new type, which represents a list of elements of that type in this case. Thus *List* is a *type-constructor*. It has two data-constructors, i.e. there are two ways to create elements of the type *List a*, for any type a. -- 'Empty' is a value of type 'List a', for all types 'a'. This is one possible way to construct elements of type 'List a'. Empty :: List a -- 'Add' is a function that takes an element of type 'a', and a 'List a', and constructs a 'List a' out of them. Add :: a -> List a -> List a Using the GHC extension, *GADTs*, it is possible to write code that directly reflects this structure. Although that is considered an advanced feature of the language. {-# LANGUAGE GADTs #-} data List a where Empty :: List a Add :: a -> List a -> List a On 23 September 2015 at 13:21, goforgit . wrote:

I have a follow up question, I hope it's alright.

What about the following?

data List a = Empty | Add a (List a)

What does the a mean and why is it possible to put it there?

On Tue, Sep 22, 2015 at 4:54 PM, goforgit . wrote:

...

Thanks Brandon, that cleared things up!

On Mon, Sep 21, 2015 at 8:39 PM, Brandon Allbery wrote:

...

On Mon, Sep 21, 2015 at 2:23 PM, goforgit . wrote:

...

Could someone explain to me the difference between the following?

data Atype = Numeric Integer | A | B C

and

data Atype = Integer | A | B C

The second one is an enumeration with three values: "Integer" (which is *not* an integer, nor does it contain one), "A", and "B" which as written there takes a value of some unspecified type C as a parameter. The first one is a enumeration with three values: "Numeric" which takes an Integer as a parameter, "A", and "B" which takes a value of some unspecified type C as a parameter.

Note that the "Integer" in the second one has *nothing whatsoever* to do with the *type* Integer.

Remember that you must always provide a data constructor with "data"; you cannot simply say "data MyInt = Integer" to "wrap" an Integer, because you have not said what to wrap it *in*. (You may have intended to create a type alias, though; that would be "type", not "data".) A "data" always requires a data constructor name, so the compiler can tell when you are talking about a value of that type by looking for the constructor.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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-- Regards Sumit Sahrawat

Thank you guys, very good descriptions given on my question. Together with your answers and the analogy of it being somewhat similar to something in C++ made me understand how it works. Again many thanks! On Wed, Sep 23, 2015 at 10:48 AM, Kim-Ee Yeoh wrote:

On Wed, Sep 23, 2015 at 2:51 PM, goforgit . wrote:

...

What about the following?

data List a = Empty | Add a (List a)

What does the a mean and why is it possible to put it there?

In addition to the good answers already given, it helps to think of it this way:

Here's a list of Bools:

data ListBool = EmptyListBool | AddListBool Bool ListBool

Here's a list of Chars:

data ListChar = EmptyListChar | AddListChar Char ListChar

Here's a list of Ints:

data ListInt = EmptyListInt | AddListInt Int ListInt

Well just look at all that repetition!

Surely there must be a way to keep DRY and abstract over all that?

Let's see: what's common to all of the above? What stays the same? What changes?

Here's something that tries to express and separate out what's "fixed" and what's "insert type here":

data ListX = Empty | Add X ListX

We're close.

That almost but doesn't quite work, because Haskell treats capital X as a concrete type, like Bool and Char and Int.

What we want is a type _variable_. And Haskell gives us that, if we use lower-case letters:

data List x = Empty | Add x (List x)

The parens is needed to distinguish against

data List x = Empty | Add x List x

which doesn't work for reasons you can probably guess.

Finally, it's convention to use type variables a b c and not x y z.

-- Kim-Ee

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On Fri, Sep 25, 2015 at 2:10 PM, goforgit . wrote:

Is it possible to make the list an instance of something in order to use for example takeWhile on List?

Not presently for takeWhile; it's specific to the built-in list type. Things are gradually being migrated to Foldable and Traversable, though. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net