2008/10/6 Galchin, Vasili
ok ... by using "newtype", we are constricting/constraining to a subset of CInt .. e.g. something like a "subtype" of CInt?? (where by "subtype", I mean like the notion of subtype in languages like Ada). For our audience, can you perhaps distinguish (in a typeful way) between the Haskell notion of "type", "newtype" and "data"? Or maybe let's distinguish between these notions not only in a typeful manner, but also in a historical motivation? .. ... motivations are always IMO very, very enlightening!
Here's an example of using newtypes: module Main where newtype Flag = Flag Int -- These are the only legal values: flag1 :: Flag flag1 = Flag 1 flag2 :: Flag flag2 = Flag 2 fun :: Int -> Flag -> Int fun n (Flag f) = undefined -- Implementation goes here. -- Using `fun`. main = do print (fun 10 flag1) -- Oh noes, the programmer messed up and reordered the -- arguments! print (fun flag1 10) Saved by the type checker: /home/tibell/Test.hs:21:13: Couldn't match expected type `Int' against inferred type `Flag' In the first argument of `fun', namely `flag1' In the first argument of `print', namely `(fun flag1 10)' In the expression: print (fun flag1 10) Failed, modules loaded: none. By creating a module that doesn't export the constructor used to create e.g. `Flag` and only the constants `flag1` and `flag2` we can make sure that no one ever calls `fun` with an illegal value. I hope this helps. Cheers, Johan