Restricting integers in a type?

Colin Paul Adams

28 Dec 2008 28 Dec '08

9:07 a.m.

Hello, I want to declare a type thus: type Coordinate = (Int, Int) But the two integers must be confined to the inclusive range 0-11. Can i express that in the type system? -- Colin Adams Preston Lancashire

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Ertugrul Soeylemez

28 Dec 28 Dec

3:26 p.m.

Colin Paul Adams wrote:

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I want to declare a type thus:

type Coordinate = (Int, Int)

But the two integers must be confined to the inclusive range 0-11. Can i express that in the type system?

Well, the type system allows you to create your own ranged integer type. Here is a rather inflexible, but working method to do it: newtype Int12 = Int12 Int deriving (Eq, Read, Show) instance Num Int12 where Int12 x + Int12 y | x+y <= 11 = Int12 (x+y) | otherwise = error "Int12 addition out of range" ... Greets, Ertugrul. -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://blog.ertes.de/

Colin Paul Adams

3:56 p.m.

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"Ertugrul" == Ertugrul Soeylemez writes:

Ertugrul> Colin Paul Adams wrote: >> I want to declare a type thus: >> >> type Coordinate = (Int, Int) >> >> But the two integers must be confined to the inclusive range >> 0-11. Can i express that in the type system? Ertugrul> Well, the type system allows you to create your own Ertugrul> ranged integer type. Here is a rather inflexible, but Ertugrul> working method to do it: Ertugrul> newtype Int12 = Int12 Int deriving (Eq, Read, Show) Ertugrul> instance Num Int12 where Int12 x + Int12 y | x+y <= 11 Ertugrul> = Int12 (x+y) | otherwise = error "Int12 addition out of Ertugrul> range" ... That would seem to allow x = -3 and y = 13, for instance. -- Colin Adams Preston Lancashire

Ertugrul Soeylemez

10:47 p.m.

Colin Paul Adams wrote:

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Ertugrul> Well, the type system allows you to create your own Ertugrul> ranged integer type. Here is a rather inflexible, but Ertugrul> working method to do it:

Ertugrul> newtype Int12 = Int12 Int deriving (Eq, Read, Show)

Ertugrul> instance Num Int12 where Int12 x + Int12 y | x+y <= 11 Ertugrul> = Int12 (x+y) | otherwise = error "Int12 addition out of Ertugrul> range" ...

That would seem to allow x = -3 and y = 13, for instance.

Not if you disallow an x = -3 and y = 13 to happen in the first place. Haskell's module and type system allows you to do that. Greets, Ertugrul. -- Key-ID: E5DD8D11 "Ertugrul Soeylemez " FPrint: 0F12 0912 DFC8 2FC5 E2B8 A23E 6BAC 998E CE40 2012 Keysrv: hkp://subkeys.pgp.net/ -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://blog.ertes.de/

Colin Paul Adams

29 Dec 29 Dec

6:37 a.m.

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"Ertugrul" == Ertugrul Soeylemez writes:

>> That would seem to allow x = -3 and y = 13, for instance. Ertugrul> Not if you disallow an x = -3 and y = 13 to happen in Ertugrul> the first place. Haskell's module and type system Ertugrul> allows you to do that. Can you explain how to do that please? -- Colin Adams Preston Lancashire

Ertugrul Soeylemez

30 Dec 30 Dec

9:13 a.m.

Colin Paul Adams wrote:

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"Ertugrul" == Ertugrul Soeylemez writes:

>> That would seem to allow x = -3 and y = 13, for instance.

Ertugrul> Not if you disallow an x = -3 and y = 13 to happen in Ertugrul> the first place. Haskell's module and type system Ertugrul> allows you to do that.

Can you explain how to do that please?

Sure: module Even (Even) where newtype Even a = Even a deriving (Eq, Show) instance Integral a => Num (Even a) where Even a + Even b = Even (a+b) -- ... fromInteger x = if even x then Even (fromInteger x) else undefined The only way to introduce incorrect Even values would be to access the constructor directly, but in that example, it's not exported. The interface to constructing Even values is the fromInteger function. Greets, Ertugrul. -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://blog.ertes.de/

Colin Paul Adams

9:15 a.m.

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"Ertugrul" == Ertugrul Soeylemez writes:

Ertugrul> Colin Paul Adams wrote: >> >>>>> "Ertugrul" == Ertugrul Soeylemez writes: >> >> >> That would seem to allow x = -3 and y = 13, for instance. >> Ertugrul> Not if you disallow an x = -3 and y = 13 to happen in Ertugrul> the first place. Haskell's module and type system Ertugrul> allows you to do that. >> >> Can you explain how to do that please? Ertugrul> Sure: Ertugrul> module Even (Even) where Ertugrul> newtype Even a = Even a deriving (Eq, Show) Ertugrul> instance Integral a => Num (Even a) where Even a + Ertugrul> Even b = Even (a+b) -- ... fromInteger x = if even x Ertugrul> then Even (fromInteger x) else undefined Ertugrul> The only way to introduce incorrect Even values would be Ertugrul> to access the constructor directly, but in that example, Ertugrul> it's not exported. The interface to constructing Even Ertugrul> values is the fromInteger function. Thanks for all your help. -- Colin Adams Preston Lancashire

Thomas Davie

28 Dec 28 Dec

4:08 p.m.

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Hello,

I want to declare a type thus:

type Coordinate = (Int, Int)

But the two integers must be confined to the inclusive range 0-11. Can i express that in the type system? -- Colin Adams Preston Lancashire _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Colin Paul Adams
Ertugrul Soeylemez
Thomas Davie