Haskell – simple constructor comparison (?) function

by

Take a look at Data.Data and its toConstr function. This returns a representation of the constructor which can be compared for equality.

With an extension (you can put {-# LANGUAGE DeriveDataTypeable #-} at the top of your module), you can have a Data instance derived for you automatically:

data PhpValue = VoidValue | IntValue Integer | BoolValue Bool 
              deriving (Typeable, Data)

You should then be able to use the toConstr function to compare by constructor.

Now the following will be true:

toConstr (BoolValue True) == toConstr (BoolValue False)

Using on from Data.Function you can now rewrite sameConstructor to:

sameConstructor = (==) `on` toConstr

This is the same as

sameConstructor l r = toConstr l == toConstr r

I think the version using on is easier to read at a glance.

More Related Contents:

  1. How to run a for loop in Haskell
  2. What is a monad?
  3. Getting started with Haskell
  4. How is this fibonacci-function memoized?
  5. runST and function composition
  6. Explain this chunk of haskell code that outputs a stream of primes
  7. How to create a polyvariadic haskell function?
  8. What does (f .) . g mean in Haskell?
  9. foldl versus foldr behavior with infinite lists
  10. What is the purpose of Rank2Types?
  11. Why does length return 1 for a tuple with 2 elements, and gives an error for a tuple with more elements?
  12. What are paramorphisms?
  13. What indentation is required for a case statement within a let statement?
  14. Generating Fibonacci numbers in Haskell?
  15. Euler 43 – is there a monad to help write this list comprehension?
  16. In Haskell, when do we use in with let?
  17. Pseudo-quicksort time complexity
  18. How to increment a variable in functional programming?
  19. What are free monads?
  20. Zipper Comonads, Generically
  21. Why is GHC complaining about non-exhaustive patterns?
  22. foldl Implementation with Runtime Errors
  23. Exception handling in Haskell
  24. How to make a type with restrictions
  25. Given a Haskell type signature, is it possible to generate the code automatically?
  26. Why does OCaml sometimes require eta expansion?
  27. Does Haskell have return type overloading?
  28. Explanation of “tying the knot”
  29. What is predicativity?
  30. Return specific type within Haskell

Leave a Comment