On 8/9/07, Henning Thielemann
On Wed, 8 Aug 2007, Samuel Bronson wrote:
Hi. I wrote a module and dons suggested I ask you guys for some tips. Here's a good deal of it:
module Data.TupleFields where
import Data.Tuple
class Field1 t f | t -> f where field1 :: t -> f field1_u :: (f -> f) -> (t -> t) field1_s :: f -> (t -> t) field1_s x = field1_u (const x)
Somewhen in the past I proposed a different usage of the field names of Haskell record fields: http://www.haskell.org/haskellwiki/Record_access Translated to the tuple issue, I use only one function of type field1 :: f -> t -> (f,t) which is a combination of 'get' and 'set'. Using this function you can implement a generic 'set', 'get', and 'update'. Of course, you can argue that it is bad style to put the distinct 'set' and 'get' functionalities into one function. Nevertheless, I think 'field1_u' should not be a class method, but there should be a separate 'update' function which combines 'field1_s' and 'field1'. I would also rename 'field' to 'field_g' or better use 'field1get', 'field1set', 'field1update' or 'field1modify' (in analogy to State monad from MTL).
Hmm. How about getField1, setField1, modifyField1 etc.? Or see below.
type Accessor r a = a -> r -> (a, r)
{- | Set the value of a field. -} set :: Accessor r a -> a -> r -> r set f x = snd . f x
{- | Get the value of a field. -} get :: Accessor r a -> r -> a get f = fst . f undefined
{- | Transform the value of a field by a function. -} modify :: Accessor r a -> (a -> a) -> (r -> r) modify f g rOld = let (a,rNew) = f (g a) rOld in rNew
Hmm, these names are a bit *too* close to the ones used in the mtl ;-). Also, are you going to make a package implementing this basic interface (preferably with names that don't conflict with mtl), or should I include it in another module in my package?