Automatically generate sum type members -> [String]

Cody Goodman

18 May 2015 18 May '15

7:20 p.m.

Can I use either template Haskell, GHC Generics, or something else to get the list of Constructors in Codes? Then I could do `map show codeSumTypeMembers` and get `["A0100A","A0500A"]`? Here's an example of solving this problem manually: data Codes = A0100A | A0500A deriving Show codeExists "A0100A" = True codeExists "A0500A" = True codeExists _ = False main = print $ codeExists xmlElementName where xmlElementName = "A0500A" -- λ> main -- True

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David Kraeutmann

18 May 18 May

7:27 p.m.

New subject: Automatically generate sum type members -> [String]

readConstr [1] from Data.Data will do the trick. DataType for a type can be retrieved with dataTypeOf. ---- 1: https://hackage.haskell.org/package/base-4.8.0.0/docs/Data-Data.html#v:readC... On 5/19/2015 1:20 AM, Cody Goodman wrote:

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Can I use either template Haskell, GHC Generics, or something else to get the list of Constructors in Codes? Then I could do `map show codeSumTypeMembers` and get `["A0100A","A0500A"]`?

Here's an example of solving this problem manually:

data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True codeExists "A0500A" = True codeExists _ = False

main = print $ codeExists xmlElementName where xmlElementName = "A0500A"

-- λ> main -- True _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Brandon Allbery

7:27 p.m.

New subject: Automatically generate sum type members -> [String]

On Mon, May 18, 2015 at 7:20 PM, Cody Goodman

...

wrote:

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Can I use either template Haskell, GHC Generics, or something else to get the list of Constructors in Codes? Then I could do `map show codeSumTypeMembers` and get `["A0100A","A0500A"]`?

Derive Enum and Bounded as well, and you can then use minBound and maxBound along with show. This will only work for nullary constructors, though. IIRC, with Data.Typeable (deriving (Typeable)) you can do something like this for non-nullary constructors as well. It's more painful, though. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

Tikhon Jelvis

7:29 p.m.

New subject: Automatically generate sum type members -> [String]

You can get this information through the Data.Data module. There are two typeclasses which can be (have to be, I believe) derived for your data type, Data and Typeable. The Data class lets you access a representation of your type including type constructors. To set this up, you need to import Data.Data and enable the DeriveDataTypeable extension: {-# LANGUAGE DeriveDataTypeable #-} import Data.Data data Codes = A0100A | A0500A deriving (Show, Data, Typeable) Now you can get a representation of the type: λ> dataTypeOf (undefined :: Codes) DataType {tycon = "Main.Codes", datarep = AlgRep [A0100A,A0500A]} Since your type is an algebraic data type (AlgRep), the DataRep contains its constructors, which you can inspect with functions from the Data.Data module. On Mon, May 18, 2015 at 4:20 PM, Cody Goodman

...

wrote:

...

Can I use either template Haskell, GHC Generics, or something else to get the list of Constructors in Codes? Then I could do `map show codeSumTypeMembers` and get `["A0100A","A0500A"]`?

Here's an example of solving this problem manually:

data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True codeExists "A0500A" = True codeExists _ = False

main = print $ codeExists xmlElementName where xmlElementName = "A0500A"

-- λ> main -- True _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Adam Bergmark

8:19 p.m.

New subject: Automatically generate sum type members -> [String]

Using GHC Generics: http://hackage.haskell.org/package/generic-deriving-1.7.0/docs/Generics-Deri... On Tue, May 19, 2015 at 1:29 AM, Tikhon Jelvis wrote:

...

You can get this information through the Data.Data module. There are two typeclasses which can be (have to be, I believe) derived for your data type, Data and Typeable. The Data class lets you access a representation of your type including type constructors.

To set this up, you need to import Data.Data and enable the DeriveDataTypeable extension:

{-# LANGUAGE DeriveDataTypeable #-} import Data.Data

data Codes = A0100A | A0500A deriving (Show, Data, Typeable)

Now you can get a representation of the type:

λ> dataTypeOf (undefined :: Codes) DataType {tycon = "Main.Codes", datarep = AlgRep [A0100A,A0500A]}

Since your type is an algebraic data type (AlgRep), the DataRep contains its constructors, which you can inspect with functions from the Data.Data module.

On Mon, May 18, 2015 at 4:20 PM, Cody Goodman < codygman.consulting@gmail.com> wrote:

...
Can I use either template Haskell, GHC Generics, or something else to get the list of Constructors in Codes? Then I could do `map show codeSumTypeMembers` and get `["A0100A","A0500A"]`?

Here's an example of solving this problem manually:

data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True codeExists "A0500A" = True codeExists _ = False

main = print $ codeExists xmlElementName where xmlElementName = "A0500A"

-- λ> main -- True _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Geraldus

19 May 19 May

2:31 a.m.

New subject: Automatically generate sum type members -> [String]

Also there is good article by Chris Done: http://chrisdone.com/posts/data-typeable. Hope this will be helpful. вт, 19 мая 2015 г. в 5:20, Adam Bergmark :

...

Using GHC Generics: http://hackage.haskell.org/package/generic-deriving-1.7.0/docs/Generics-Deri...

On Tue, May 19, 2015 at 1:29 AM, Tikhon Jelvis wrote:

...
You can get this information through the Data.Data module. There are two typeclasses which can be (have to be, I believe) derived for your data type, Data and Typeable. The Data class lets you access a representation of your type including type constructors.

To set this up, you need to import Data.Data and enable the DeriveDataTypeable extension:

{-# LANGUAGE DeriveDataTypeable #-} import Data.Data

data Codes = A0100A | A0500A deriving (Show, Data, Typeable)

Now you can get a representation of the type:

λ> dataTypeOf (undefined :: Codes) DataType {tycon = "Main.Codes", datarep = AlgRep [A0100A,A0500A]}

Since your type is an algebraic data type (AlgRep), the DataRep contains its constructors, which you can inspect with functions from the Data.Data module.

On Mon, May 18, 2015 at 4:20 PM, Cody Goodman < codygman.consulting@gmail.com> wrote:

...
Can I use either template Haskell, GHC Generics, or something else to get the list of Constructors in Codes? Then I could do `map show codeSumTypeMembers` and get `["A0100A","A0500A"]`?

Here's an example of solving this problem manually:

data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True codeExists "A0500A" = True codeExists _ = False

main = print $ codeExists xmlElementName where xmlElementName = "A0500A"

-- λ> main -- True _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Anatoly Zaretsky

5:03 a.m.

New subject: Automatically generate sum type members -> [String]

On Tue, May 19, 2015 at 2:20 AM, Cody Goodman wrote:

...

data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True codeExists "A0500A" = True codeExists _ = False

data Codes = A0100A | A0500A deriving Read codeExists code = case reads code of [(_, "")] -> True _ -> False Or with -XPatternGuards: codeExists code | [(_, "")] <- reads code = True | otherwise = False

Anatoly Zaretsky

5:12 a.m.

New subject: Automatically generate sum type members -> [String]

On Tue, May 19, 2015 at 12:03 PM, Anatoly Zaretsky wrote:

...

data Codes = A0100A | A0500A deriving Read

codeExists code = case reads code of [(_, "")] -> True _ -> False

Or with -XPatternGuards:

codeExists code | [(_, "")] <- reads code = True | otherwise = False

Oops, should have checked the types before posting: codeExists code = case reads code :: [(Codes, String)] of [(_, "")] -> True _ -> False Otherwise the compiler could not guess the right Read instance.

Joachim Breitner

5:15 a.m.

Hi, Am Dienstag, den 19.05.2015, 12:03 +0300 schrieb Anatoly Zaretsky:

...

On Tue, May 19, 2015 at 2:20 AM, Cody Goodman wrote:

...
data Codes = A0100A | A0500A deriving Show

codeExists "A0100A" = True codeExists "A0500A" = True codeExists _ = False

data Codes = A0100A | A0500A deriving Read

codeExists code = case reads code of [(_, "")] -> True _ -> False

Or with -XPatternGuards:

codeExists code | [(_, "")] <- reads code = True | otherwise = False

since base-4.6, there is readMaybe :: Read a => String -> Maybe a in Text.Read. So it might be nicer to write codeExists code = isJust (readMaybe code :: Maybe Codes) Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org

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Adam Bergmark
Anatoly Zaretsky
Brandon Allbery
Cody Goodman
David Kraeutmann
Geraldus
Joachim Breitner
Tikhon Jelvis

Automatically generate sum type members -> [String]

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