Define a primitive data type: Difference between revisions

Haskell doesn't have any built-in subrange types. However, it is possible to declare arbitrary types that "behave like" any of the built-in types on the "usual" numeric etc. operations, because these operations are defined by type-classes. So we generalize the task a bit, and first declare a generic container type that supports an additional ''check'' operation. Then, we lift any operation in the base type to the container type, by executing the check after each operation:

 

data Check a b = Check { unCheck :: b } deriving (Eq, Ord)

quotRem = lift2p quotRem

divMod = lift2p divMod

 

Now we can declare the a subrange 1..10 of integer like this:

instance Checked TinyInt Int where

check x | x >= 0 && x <= 10 = Check x

 

In the same way, we could now declare the subtype of the even integers:

 

instance Checked EvenInt Int where

check x | even x = Check x

 

 

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