Declaring a tuple of instances of Enums as an instance of the Enum class

R J

23 May 2010 23 May '10

9:18 a.m.

Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input 'deriving'". Why is this error generated? Is there a syntax error, or is there a conceptual problem with enumerating a Cartesian product, such as Month x Month? The cardinality of the Cartesian product is finite (including the bottom values, cardinality = 1 + (12 + 1)*(12 + 1) = 170), and so the product is amenable at least to some arbitrary enumeration (such as Cantor's diagonal method). Thanks. _________________________________________________________________ The New Busy think 9 to 5 is a cute idea. Combine multiple calendars with Hotmail. http://www.windowslive.com/campaign/thenewbusy?tile=multicalendar&ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_5

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Ivan Lazar Miljenovic

23 May 23 May

9:33 a.m.

New subject: Declaring a tuple of instances of Enums as an instance of the Enum class

R J writes:

...

Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input deriving'".

You can't derive instances for type aliases (as its mainly there for documentation reasons, etc.). However, pairs don't have Enum instances by default so you still can't use its instance. If you define "data MonthPair = MonthPair Month Month" then you should be able to derive Enum. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

Daniel Fischer

9:51 a.m.

New subject: Declaring a tuple of instances of Enums as an instance of the Enum class

On Sunday 23 May 2010 15:33:58, Ivan Lazar Miljenovic wrote:

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R J writes:

...
Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input deriving'".

You can't derive instances for type aliases (as its mainly there for documentation reasons, etc.). However, pairs don't have Enum instances by default so you still can't use its instance.

If you define "data MonthPair = MonthPair Month Month" then you should be able to derive Enum.

No, per the report (http://haskell.org/onlinereport/derived.html) "Derived instance declarations for the class Enum are only possible for enumerations (data types with only nullary constructors)." You can derive Enum instances for newtype wrappers around Enums with GHC (possibly others), but for types such as MonthPair you have to give the instances yourself (maybe you can let them be generated by tools like Derive).

Ivan Lazar Miljenovic

9:59 a.m.

New subject: Declaring a tuple of instances of Enums as an instance of the Enum class

Daniel Fischer writes:

...

On Sunday 23 May 2010 15:33:58, Ivan Lazar Miljenovic wrote:

...
R J writes:

...
Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input deriving'".

You can't derive instances for type aliases (as its mainly there for documentation reasons, etc.). However, pairs don't have Enum instances by default so you still can't use its instance.

If you define "data MonthPair = MonthPair Month Month" then you should be able to derive Enum.

No, per the report (http://haskell.org/onlinereport/derived.html)

"Derived instance declarations for the class Enum are only possible for enumerations (data types with only nullary constructors)."

Whoops, you're right. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

Henning Thielemann

25 May 25 May

2:30 p.m.

New subject: Declaring a tuple of instances of Enums as an instance of the Enum class

Daniel Fischer schrieb:

...

On Sunday 23 May 2010 15:33:58, Ivan Lazar Miljenovic wrote:

...
R J writes:

...
Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input deriving'". You can't derive instances for type aliases (as its mainly there for documentation reasons, etc.). However, pairs don't have Enum instances by default so you still can't use its instance.

If you define "data MonthPair = MonthPair Month Month" then you should be able to derive Enum.

No, per the report (http://haskell.org/onlinereport/derived.html)

"Derived instance declarations for the class Enum are only possible for enumerations (data types with only nullary constructors)."

You might define an instance more generally: data EnumPair a b = EnumPair a b instance (Enum a, Bounded b, Enum b) => Enum (EnumPair a b) where ... Then define something like fromEnum (EnumPair a b) = (maxBound - minBound)*a + b (needs still some asTypeOf's and fromEnum's)

Ryan Ingram

9:47 p.m.

New subject: Declaring a tuple of instances of Enums as an instance of the Enum class

Also, this way lies a bit of madness, since fromEnum/toEnum work on Int, not Integer. This means EnumPair (EnumPair Month Month) (EnumPair Month Month) overflows -- ryan On Tue, May 25, 2010 at 11:30 AM, Henning Thielemann wrote:

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Daniel Fischer schrieb:

...
On Sunday 23 May 2010 15:33:58, Ivan Lazar Miljenovic wrote:

...
R J writes:

...
Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input deriving'". You can't derive instances for type aliases (as its mainly there for documentation reasons, etc.). However, pairs don't have Enum instances by default so you still can't use its instance.

If you define "data MonthPair = MonthPair Month Month" then you should be able to derive Enum.

No, per the report (http://haskell.org/onlinereport/derived.html)

"Derived instance declarations for the class Enum are only possible for enumerations (data types with only nullary constructors)."

You might define an instance more generally:

data EnumPair a b = EnumPair a b

instance (Enum a, Bounded b, Enum b) => Enum (EnumPair a b) where ...

Then define something like fromEnum (EnumPair a b) = (maxBound - minBound)*a + b

(needs still some asTypeOf's and fromEnum's)

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Daniel Fischer
Henning Thielemann
Ivan Lazar Miljenovic
R J
Ryan Ingram