On 12 December 2012 21:57, Navid Hallajian <navi...@gmail.com> wrote: > Hello, > > I'm a beginner in Haskell, so forgive me if this is a basic question, but > I'd like to know if it's possible to have a predicate as part of a data > type, so that when the data type is created, it can only be done if it > satisfies the predicate else a type error is thrown. > > For instance, a matrix with integer elements could be modelled as [[Int]], > given the restrictions that > > it must have at least one column and one row (so there must be at least one > list), and > every list must have the same length > > so that when a matrix is created, the type system wont allow it if the > predicates aren't met.
Write a custom constructor function that returns a Maybe: newtype Matrix = Matrix [[Int]] makeMatrix :: [[Int]] -> Maybe Matrix makeMatrix [] = Nothing -- No columns makeMatrix [[]] = Nothing -- Has a column but no rows ... It is possible with smarter types to encode various predicates into the actual type, but for your purposes they probably aren't required. > > Thanks, > > Navid > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com http://IvanMiljenovic.wordpress.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
