[Haskell-begin] data declarations
hi Not sure if I'm on the right track here, so as a beginner I thought I'd ask on this list. I want to declare a type 'All' that consists of the of the types 'Tinteger, Tbool, Tstring, Tarray' but I would also like to make a relationship that Tinteger and Tbool are also the type 'Simple' and that 'Tstring' and 'Tarray' are 'Composite' and that Mixed is then either Simpletype or Composite. So I can then write a function that takes a type of All and produces a list of Mixed. data All = Tinteger Int | Tbool Bool | Tstring String | Tarray [Alltypes] data Simple = Tinteger | Tbool data Composite = Tstring | Tarray data Mixed = Simple | Composite k :: All -> [Mixed] k a = case a of Simple s -> [Simple s] Composite c -> [Composite c] But this isn't valid Haskell code because I can't have multiple declarations of the same types, is there a way to specify a relationship, if so can someone point me in the right direction? Roger
On 2008 Jul 22, at 23:01, Roger Myers wrote:
Not sure if I'm on the right track here, so as a beginner I thought I'd ask on this list.
I want to declare a type 'All' that consists of the of the types 'Tinteger, Tbool, Tstring, Tarray' but I would also like to make a relationship that Tinteger and Tbool are also the type 'Simple' and that 'Tstring' and 'Tarray' are 'Composite' and that Mixed is then either Simpletype or Composite.
So I can then write a function that takes a type of All and produces a list of Mixed.
Any particular reason that All can't be represented by Mixed? That would be my first cut at it (assuming I didn't need advanced type hackery for some reason). -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
Brandon S. Allbery KF8NH
On 2008 Jul 22, at 23:01, Roger Myers wrote:
Not sure if I'm on the right track here, so as a beginner I thought I'd ask on this list.
I want to declare a type 'All' that consists of the of the types 'Tinteger, Tbool, Tstring, Tarray' but I would also like to make a relationship that Tinteger and Tbool are also the type 'Simple' and that 'Tstring' and 'Tarray' are 'Composite' and that Mixed is then either Simpletype or Composite.
So I can then write a function that takes a type of All and produces a list of Mixed.
Any particular reason that All can't be represented by Mixed? That would be my first cut at it (assuming I didn't need advanced type hackery for some reason).
Hmm... ah, bad thinking on my part. I think this serves me a lot better: data Simple = Tinteger Int | Tbool Bool data Composite = Tstring String | Tarray [All] data All = Simple | Composite k :: All -> [All] k a = case a of Simple -> [a] Composite -> [a] Thanks :)
On Wed, Jul 23, 2008 at 5:42 AM, Jeff Zaroyko
data Simple = Tinteger Int | Tbool Bool data Composite = Tstring String | Tarray [All]
data All = Simple | Composite
Don't you mean: data All = Simple Simple | Composite Composite So the 'All' type contains the 'Simple' and 'Composite' types. In case you find it confusing that the constructor names are the same as the names of the types you can write something like: data All = S Simple | C Composite I don't know what your 'k' should do. Because your function...
k :: All -> [All] k a = case a of Simple -> [a] Composite -> [a]
...can be simplified to: 'k a = [a]' (if you disregard the strictness properties) which isn't very useful. So I guess you mean something different. regards, Bas
participants (4)
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Bas van Dijk -
Brandon S. Allbery KF8NH -
Jeff Zaroyko -
Roger Myers