You can code array types with static bounds with the existing Haskell type system.
On Sun, Aug 17, 2008 at 3:45 PM, Ramin <[EMAIL PROTECTED]> wrote: > I am new to both the Haskell language, and to this group, but I have > recently become obsessed with the mathematical precision of Haskell, and I > want to help improve it, if members of the group could bear with me. > > The one thing I dislike about the Haskell language is it's treatment of > arrays. I don't understand how things work internally to the system, and > perhaps array-manipulating code can be efficiently optimized, but I would > prefer to have a language feature for explicitly creating and modifying > arrays in a way that does not require the entire array be copied on every > update. > > My idea is this: a fixed-width mutable array can be declared as a type, much > like a list, but can be evaluated more like a bitwise-integer operation. > > -- in an array of A's set the 5th item in the array with an "initial-A" > value > changeArrayFunc :: a^10 -> a^10 > changeArrayFunc ar = (ar:5 <- initA) -- returns an array which is that > same as the old array, except the value at index 5 is different. > > -- select the 5th item of an array > getArrayItemFunc :: a^10 -> a > getArrayItemFunc ar = (ar:5) > > Here, I use the caret (^) operator to indicate an "array-10" type. I used > caret because it is typically used for exponents -- as in, if your data type > has N possible values, an array with 10 units would have N^10 possible > values. Then, I use the colon (:) operator to do indexing. I've seen the > various proposals for indexing operators and honestly I don't care which > operator is chosen; I just use the colon operator because that is how lists > are treated in pattern matching. > > The important thing is that an array-type exists, that way all > bounds-checking can be done at compile-time by the typing system. Using > arrays of different lengths would result in a compile-time error: Obviously, > this will increase the complexity of the typing system. > > data Something = Array Int^10 > > change5Array :: Int^5 -> Int^5 > change5Array ar = ((ar:4) <- 0) > > -- Something has an array of type Int^10, but calls "change5Array" which > expects an Int^5 > badFunc :: Something -> Int > badFunc (Array x) = (change5Array x) > > -- COMPILE-TIME ERROR > -- Arrays.hs:8:16: > -- Couldn't match expected type `Int^5' against inferred type `Int^10' > -- In the first argument of `change5array', namely `x' > -- In the expression: (change5array x) > -- In the definition of `badFunc': > -- badFunc (Array x) = (change5Array x) > > ...or something like that. > > An efficient implementation of array access would make Haskell very useful > for a much wider variety of computationally intensive applications. Haskel > could be used to efficiently model memory access, provided that the > interpreter knew not to "copy" arrays upon update, but simply to update a > value within an array. If arrays were a language feature of Haskell, then > this optimization could be guaranteed. > > If anyone takes the time to consider this idea, or to tell my why this isn't > necessary, I would be most greatful. > > -- Ramin Honary > > _______________________________________________ > Haskell-prime mailing list > Haskell-prime@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-prime > _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
