[Haskell-cafe] Matching constructors
Hi, If I have something like data Patootie = Pa Int | Tootie Int and I want to pull out the indices of all elements of a list that have type constructor Tootie, how would I do that? I thought I might be able to use findIndices, but I don't know how to express the predicate. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Matching constructors
tootieIndices = findIndices isTootie where isTootie (Pa _) = False isTootie (Tootie _) = True would be my first approach. /g On 2/10/06, Creighton Hogg [EMAIL PROTECTED] wrote: Hi, If I have something like data Patootie = Pa Int | Tootie Int and I want to pull out the indices of all elements of a list that have type constructor Tootie, how would I do that? I thought I might be able to use findIndices, but I don't know how to express the predicate. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- We have lingered in the chambers of the sea By sea-girls wreathed with seaweed red and brown Till human voices wake us, and we drown. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Matching constructors
On Fri, 10 Feb 2006, Creighton Hogg wrote:
Hi,
If I have something like
data Patootie = Pa Int | Tootie Int
and I want to pull out the indices of all elements of a list
that have type constructor Tootie, how would I do that?
I thought I might be able to use findIndices, but I don't
know how to express the predicate.
(\p - case p of {Pa _ - False; Tootie _ - True})
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Re: [Haskell-cafe] Matching constructors
Or inline as findIndices (\x - case x of Tootie _ - True; _ - False) listOfPasAndTooties There was a recent thread about wanting a more succint way to write this (unary pattern matching): http://thread.gmane.org/gmane.comp.lang.haskell.cafe/11109 If John got his wish, then you could write something like findIndices (@ Tootie _) listOfPasAndTooties Maybe this feature will appear in a future Haskell standard? though I don't see anything on the Haskell' wiki about this... Cheers Jared. -- http://www.updike.org/~jared/ reverse )-: ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Matching constructors
Creighton Hogg [EMAIL PROTECTED] writes: data Patootie = Pa Int | Tootie Int and I want to pull out the indices of all elements of a list that have type constructor Tootie, how would I do that? x = [Pa 3, Tootie 5, Pa 7, Tootie 9, Pa 11] y = [ i |Tootie i - x ] z = [ i | i@(Tootie _) - x ] y or z might be helpful. -- Mark ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Matching constructors
On 10/02/06, Creighton Hogg [EMAIL PROTECTED] wrote:
Hi,
If I have something like
data Patootie = Pa Int | Tootie Int
and I want to pull out the indices of all elements of a list
that have type constructor Tootie, how would I do that?
I thought I might be able to use findIndices, but I don't
know how to express the predicate.
Just to clear up a small point, Tootie isn't a type constructor, but a
data constructor. ('Maybe' is a type constructor, 'Just' is a data
constructor.)
You can use list comprehensions with pattern matching to write this
fairly succinctly:
tootieIndices xs = [i | (Tootie {}, i) - zip xs [0..]]
The {} matches whatever parameters Tootie might have, so this will
continue to work even if you later extend the Tootie data constructor
with more fields. If you want to match more carefully, you can of
course put a variable there.
- Cale
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Re: [Haskell-cafe] matching constructors
On Mar 5, 2004, at 15:48, Vadim Zaliva wrote: OK, I figured it out. For sake of other novices like me here is what you need to do to make it work. 0. Need to import Data.Generics 1. Compile with '-fglasgow-exts' flag 2. When deriving from Data you also need to derive from Typeable. It slightly bothers me that this solution seems to be using non-standard GHC extensions. Vadim On Mar 5, 2004, at 12:41, Brandon Michael Moore wrote: At the lower level of remimplementing your functions I can suggest a few things. Brandon, Thanks for great suggestions! Following them, here is how I redone the code: ... import Data.Maybe import Data.Either import Data.Typeable ... data Flag = Verbose | Input String | Output String | Filter String deriving Show Data instance Eq Flag where x == y = toConstr x == toConstr y findFlag :: Flag - [Flag] - Flag findFlag f fs = fromMaybe f (find (==f) fs) The only problem is my GHC does not seems to find 'Data' class and I am getting following errors: Type constructor or class not in scope: `Data' Variable not in scope: `toConstr' Variable not in scope: `toConstr' Also I have style question: What is the best way to define equality test in this example: 1. Via instantiating EQ class 2. via standalone function (I can define sameConstr Flag - Flag - Bool) 3. inline lambda expression passed to find I am leaning towards #2 or #3. Vadim -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) smime.p7s Description: S/MIME cryptographic signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
Vadim Zaliva wrote: [...] It slightly bothers me that this solution seems to be using non-standard GHC extensions. Hmmm, using generics seems like massive overkill for option handling. Could you describe what you are exactly trying to achieve? Cheers, S. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
On Mar 8, 2004, at 11:17, Sven Panne wrote: Hmmm, using generics seems like massive overkill for option handling. Could you describe what you are exactly trying to achieve? I am doing command line options parsing. I've defined Flag type with constructor for each possible option: data Flag = Verbose | Input String | Output String | Filter String deriving (Show, Typeable, Data) getOpt returns me a list of such objects. Now I need to look things up there by constructor. For example: doSomething fltflag where (Filter fltflag) = findFlag (Filter none) opts To achieve this I've defined: instance Eq Flag where x == y = toConstr x == toConstr y findFlag :: Flag - [Flag] - Flag findFlag f fs = fromMaybe f (find (==f) fs) Sincerely, Vadim -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) smime.p7s Description: S/MIME cryptographic signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
On Mon, 8 Mar 2004, Vadim Zaliva wrote: I am doing command line options parsing. I've defined Flag type with constructor for each possible option: data Flag = Verbose | Input String | Output String | Filter String deriving (Show, Typeable, Data) getOpt returns me a list of such objects. Now I need to look things up there by constructor. For example: doSomething fltflag where (Filter fltflag) = findFlag (Filter none) opts Try this instead: doSomething $ option none [fltflag | Filter fltflag - opts] ... option :: a - [a] - a option def [] = def option def [x] = x option def _ = error Only one of each option allowed -- Ben ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
On Mar 8, 2004, at 12:55, Ben Rudiak-Gould wrote: This would work, but I will have to write [] part for each option. Generics approach is overkill but looks much neater when used. But thanks for suggestion anyway, it is always good to learn yet another way of doing things. Sincerely, Vadim Try this instead: doSomething $ option none [fltflag | Filter fltflag - opts] option :: a - [a] - a option def [] = def option def [x] = x option def _ = error Only one of each option allowed -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) smime.p7s Description: S/MIME cryptographic signature ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] matching constructors
Hi! I am new to Haskell, but I have background in various programming languages (including Lisp) I have very basic question, if there is a way to match algebraic types constructors besides use of pattern matching. I wrote today code like this: data Flag = Verbose | Input String | Output String | Filter String deriving Show instance Eq Flag where Verbose == Verbose = True Input _ == Input _ = True Output _ == Output _ = True Filter _ == Filter _ = True _ == _ = False findFlag :: Flag - [Flag] - Flag findFlag f [] = f findFlag y (x:xs) = if x == y then x else findFlag y xs If there is a cleaner way to implement findFlag and Eq Flag? Vadim -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] matching constructors
On Fri, 5 Mar 2004, Vadim Zaliva wrote: Hi! I am new to Haskell, but I have background in various programming languages (including Lisp) I have very basic question, if there is a way to match algebraic types constructors besides use of pattern matching. I wrote today code like this: data Flag = Verbose | Input String | Output String | Filter String deriving Show instance Eq Flag where Verbose == Verbose = True Input _ == Input _ = True Output _ == Output _ = True Filter _ == Filter _ = True _ == _ = False findFlag :: Flag - [Flag] - Flag findFlag f [] = f findFlag y (x:xs) = if x == y then x else findFlag y xs If there is a cleaner way to implement findFlag and Eq Flag? Vadim It looks like you are doing some sort of option processing. There have been some suggestions recently for better approaches on the haskell list. See the thread High-level technique for program options handling from January. The basic idea there is to define a record type holding your options in the most useful form for your program, and turn your arguments into functions that transfrom such a record, rather than values you need to analyze. The example in Tomasz Zeilonka's post has exactly the option structure of your code. At the lower level of remimplementing your functions I can suggest a few things. It's probably overkill, but the Data typeclass provides an operation that takes a value and returns the constructor used. Add Data to the list of typeclasses you want to derive, and you can write your equality function as (GHC only) x == y = toConstr x == toConstr y For your findFlag function, the library function find is nearly what you want, except it returns a value in Maybe rather than taking a default: findFlag :: Flag - [Flag] - Maybe Flag findFlag f fs = find (==f) fs You could use the maybe function (Maybe a - b - (a-b) - b) to supply the default. Brandon -- La perfection est atteinte non quand il ne reste rien a ajouter, mais quand il ne reste rien a enlever. (Antoine de Saint-Exupery) ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
