An operator symbol starting with a colon is a constructor

Asked 22/1, 2015 at 11:55 Answered 22/1, 2015 at 12:24

I learn Haskell. From Haskell 2010 documentation:

An operator symbol starting with a colon is a constructor.

An operator symbol starting with any other character is an ordinary identifier.

I don't understand first phrase. I know exist data constructors and class type constructors. What constructor this case? Maybe I need a code sample.

Postgraduate answered 22/1, 2015 at 11:55 Comment(1)

An lowercase alphanumeric identifier is to an uppercase identifier like a (non-:) symbolic operator to a symbol starting with a colon. – Process 22/1, 2015 at 15:14

You can define stuff like

data Symbolic n
   = Constant n
   | Variable String
   | Symbolic n :+ Symbolic n
   | Symbolic n :* Symbolic n
  deriving (Show)

GHCi> let v = Variable; c = Constant
GHCi> c 2 :* v"a" :+ c 3
(Constant 2 :* Variable "a") :+ Constant 3

That's what the first phrase refers to.

Unshapen answered 22/1, 2015 at 11:58 Comment(3)

thank you. I don't understand two last rows in your code. Can you expand it for me? – Postgraduate 22/1, 2015 at 12:4

Try out the example in ghci. – Unshapen 22/1, 2015 at 12:8

Thanks, but I didn't understand advantage of such behavior yet. – Postgraduate 22/1, 2015 at 12:14

I know exist data constructors and class type constructors. What constructor this case?

In standard Haskell only data constructors can be symbolic and type names must be alphanumeric. If you enable the GHC extension TypeOperators, type names can be symbolic as well, allowing you to define type constructors that start with :.

Symphonia answered 22/1, 2015 at 12:24 Comment(8)

I want to try it. How can I enable the TypeOperators extension? – Postgraduate 22/1, 2015 at 12:42

@Bush You can add the -XTypeOperators flag to your call to ghc or ghci. You can also add {-# LANGUAGE TypeOperators #-} to the beginning of a Haskell file to get the same effect without any compiler options. – Symphonia 22/1, 2015 at 13:14

I add it and rename Symbolic to Symbolic#. But ghci doesn't load it. – Postgraduate 22/1, 2015 at 13:30

@Bush A name needs to either be all symbolic or all alphanumeric. And as mentioned symbolic constructor names need to start with a colon (otherwise it's a type variable). So you'd need to rename Symbolic to something like :-* or some other combination of symbols starting with a colon. Also remember that symbolic names are used infix when not enclosed in parentheses, so it'd be either data a :-* b = ... or data (:-*) n = .... – Symphonia 22/1, 2015 at 13:37

{-# LANGUAGE TypeOperators #-} data (:-) n = Constant n | Variable String | (:-) n :+ (:-) n | (:-) n :* (:-*) n deriving (Show) – Postgraduate 22/1, 2015 at 13:57

this compiles fine but don't working: λ: let n = (:+) (:-* 10) (:-* 20) <interactive>:77:15: Not in scope: data constructor :-*' Perhaps you meant :*' (line 6) – Postgraduate 22/1, 2015 at 13:58

@Bush In the code you've shown, you defined the data constructors :+ and :*, but you're trying to use :-*, which you didn't define anywhere. Something like let n :: (:-) Integer = Constant 1 :+ Constant 2 should work (the type signature is optional, of course, but this way the type constructor appears in the code). – Symphonia 22/1, 2015 at 14:14

@sepp2k, surely you meant data constructors? With TypeOperators data a +++ b = a :*** b is totally allowed, while data a +++ b = a *** b isn't. – Ninetta 1/6, 2017 at 13:30

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