Positive integer type

by 
module Positive (toPositive, getPositive, Positive) where

newtype Positive = Positive { unPositive :: Int }

toPositive :: Int -> Maybe Positive
toPositive n = if (n <= 0) then Nothing else Just (Positive n)

-- We can't export unPositive, because unPositive can be used
-- to update the field.  Trivially renaming it to getPositive
-- ensures that getPositive can only be used to access the field
getPositive :: Positive -> Int
getPositive = unPositive

The above module doesn’t export the constructor, so the only way to build a value of type Positive is to supply toPositive with a positive integer, which you can then unwrap using getPositive to access the actual value.

You can then write a function that only accepts positive integers using:

positiveInputsOnly :: Positive -> ...

More Related Contents:

  1. Why give me error? [closed]
  2. What is the difference between . (dot) and $ (dollar sign)?
  3. Haskell ranges and floats
  4. Defining a function by equations with different number of arguments
  5. :sprint for polymorphic values?
  6. Small Haskell program compiled with GHC into huge binary
  7. Multi-line commands in GHCi
  8. Writing foldl using foldr
  9. Haskell: Parse error in pattern
  10. IO happens out of order when using getLine and putStr
  11. What is indexed monad?
  12. What does $ mean/do in Haskell?
  13. Why are polymorphic values not inferred in Haskell?
  14. Cabal not installing dependencies when needing profiling libraries?
  15. Library function to compose a function with itself n times
  16. Why doesn’t Haskell sequence these IO actions properly?
  17. Why doesn’t this function work if I use “[xs]” instead of “xs”?
  18. What is fusion in Haskell?
  19. Typeclass constraints on data declarations
  20. What does the `forall` keyword in Haskell/GHC do?
  21. How does deriving work in Haskell?
  22. Foldr/Foldl for free when Tree is implementing Foldable foldMap?
  23. What characters are permitted for Haskell operators?
  24. Converting IO Int to Int
  25. Convert String to Integer/Float in Haskell?
  26. Haskell – depth for each node in binary tree using Reader monad
  27. Haskell Constraint is no smaller than the instance head
  28. Can I match a data constructor wildcard in Haskell?
  29. Is there, in Haskell, something similar to sub-guards?
  30. What is the Comonad typeclass in Haskell?

Leave a Comment