Am Donnerstag, 26. Februar 2009 06:05 schrieb Alexander Dunlap:

On Wed, Feb 25, 2009 at 4:18 PM, Philip Scott wrote:

...

months = range (Jan, Dec) : months

But of course, what you get here is a list of lists, with each element being a list of [Jan, Feb, ...., Dec]

So I puzzled for a little bit about how to do this in the most Haskelly way and I thought of this

months = concat (range (Jan, Dec) : months)

Which should work, right**

But the type checker is not pleased at all and complains:

Couldn't match expected type `[Month]' against inferred type `Month' Expected type: [[Month]] Inferred type: [Month] In the expression: concat (range (Jan, Dec) : months) In the definition of `months': months = concat (range (Jan, Dec) : months)

However, if you use the first definition and make a second function:

months = range (Jan, Dec) : months realmonths = concat(months)

The problem is that when you go from

...

months = range (Jan,Dec) : months realmonths = concat months

to

...

months = concat (range (Jan,Dec) : months)

you're not just collapsing the two functions, you're changing the definition (and type!) of "months" which you referred to within the months function. Since months is a recursive function, you can't necessary fold in the definition of realmonths because you want the recursion to apply to the original months, not the original realmonths.

As you suspected, there are better and simpler ways of doing this available, although your way is good for getting the hang of recursion, even though experienced haskellers really don't use recursion all that much. (Most prefer to use functions that encapsulate recursion *patterns* such as map, filter, folds and many more.) You could use list concatentation while keeping the recursion, as in

...

months = range (Jan,Dec) ++ months

Even better, import Data.List and use the built-in cycle function

(http://www.haskell.org/ghc/dist/current/docs/libraries/base/Data-List.html#v...):

...

months = cycle (range (Jan,Dec))

One more improvement, include also Enum among the derived classes, then you can write months = cycle [Jan .. Dec] Oh, and cycle is also exported from the Prelude, so it's not necessary to import Data.List for that (though you will probably want to imoprt it anyway).

Hope that helps.

Alex

Cheers, Daniel

Am Donnerstag, 26. Februar 2009 12:26 schrieb Philip Scott:

Hi ho,

...

One more improvement, include also Enum among the derived classes, then you can write

months = cycle [Jan .. Dec]

Oh, and cycle is also exported from the Prelude, so it's not necessary to import Data.List for that (though you will probably want to imoprt it anyway).

Ohh so many little tasty titbits of Haskell - I was trying for ages to get the '..' operator to work. I guess I will start to know where to look for these guys as time goes on.

The '..' things are syntactic sugar for enumerations: [a .. ] === enumFrom a [a, b .. ] === enumFromThen a b [a .. b] === enumFromTo a b [a, b .. c] === enumFromThenTom a b c they are methods of class Enum. :i Enum in ghci or hugs gives the class information (methods and instances currently in scope). One place to look for such things is the Haskell report, also many tutorials and books have a sectioned 'Overview of standard type classes' or some such, there you should find this and similarly important things. Another method is to ask ghci or hugs - you have to do it the right way, which is not always obvious - as in Prelude> :t \a b -> [a .. b] \a b -> [a .. b] :: (Enum t) => t -> t -> [t] so you know you have to look at Enum Prelude> :i Enum class Enum a where succ :: a -> a pred :: a -> a toEnum :: Int -> a fromEnum :: a -> Int enumFrom :: a -> [a] enumFromThen :: a -> a -> [a] enumFromTo :: a -> a -> [a] enumFromThenTo :: a -> a -> a -> [a] -- Defined in GHC.Enum instance Enum Integer -- Defined in GHC.Num instance Enum Float -- Defined in GHC.Float instance Enum Double -- Defined in GHC.Float instance Enum Bool -- Defined in GHC.Enum instance Enum Ordering -- Defined in GHC.Enum instance Enum Char -- Defined in GHC.Enum instance Enum () -- Defined in GHC.Enum instance Enum Int -- Defined in GHC.Enum looking at this, you don't see '..', but you see two functions with the correct type, that it's more likely to be enumFromTo rather than enumFromThen can easily be inferred from the functions' names.

Cheers,

Philip

Cheers, Daniel

Philip Scott writes:

Hi Alex,

...

The problem is that when you go fro

...

months = range (Jan,Dec) : months realmonths = concat months

Another way to go about this is to always look at types. You've correctly surmised that you have to refer back to the name you're defining, months, in a recusive definition, so let's write it down as months = range (Jan,Dec) `somehowJoinWith` months What is somehowJoinWith's type? Its first argument is [Month], let's call it [a]. So somehowJoinWith :: [a] -> b -> b But you want its result to be a list of Month's too. So b = [a] and somehowJoinWith :: [a] -> [a] -> [a] Cons (:) simply won't fit here, as it is :: a -> [a] -> [a]. (++) OTOH does fit, and is exactly what you need. Thinking with types helps clarify the problem always. It helps us not to confuse oranges and boxes-of-oranges. :) Cheers,