CODEWITHSUNDEEP
haskell
Nelson Tatius
Nelson Tatius

Reputation: 8043

Short == realization

I have a simple type

data Day =  Monday | Tuesday | Wednesday | Thursday | Friday

I'm a newbie in haskell, so I write == as follows.

(==) :: Day -> Day -> Bool
Monday == Monday = True
Tuesday == Tuesday = True
Wednesday == Wednesday = True
...
x == y = False

Is there any shorter way to write == realization?

Upvotes: 4

Views: 152

Answers (2)

Squidly
Squidly

Reputation: 2717

You can write

data Day =  Monday | Tuesday | Wednesday | Thursday | Friday
  deriving Eq

Which will mean that GHC will automatically generate an instance of Eq for Day. It will generate (==) such that Monday == Monday, Tuesday == Tuesday is True etc, but Monday == Friday is False

Note that you can't write something like

(==) :: Day -> Day -> Bool
x == x = True
x == y = False

which is perhaps what you were thinking of.

If you try, GHC will complain about conflicting definitions for x.

Upvotes: 6

Deestan
Deestan

Reputation: 17136

You can get the compiler to autogenerate these by using the deriving keyword:

data Day = Monday | Tuesday | Wednesday | Thursday | Friday
           deriving Eq

This will define both == and /= for your datatype.

"Eq may be derived for any datatype whose constituents are also instances of Eq." http://www.haskell.org/ghc/docs/7.4.2/html/libraries/base/Data-Eq.html

Upvotes: 12

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