[Haskell-cafe] an idea for modifiyng data/newtype syntax: use `::=` instead of `=`

[Alexey Muranov]

Hello,

i would like to suggest an idea for modifying the basic data/newtype syntax in Haskell: replace the equality sign `=` with `::=`.

When i started learning Haskell, the most confusing part of the syntax for me was the equality sign in `data` definition.  I could not even guess what the `data` definition meant without reading a chapter or two about types in Haskell, and i think it was partially due to the equality sign.  I still find this notation inconsistent with other uses of the equality sign in Haskell and in general.

For example, in

    type Name = String
    data Date = Date Int Int Int
    data Anniversary = Birthday Name Date | Wedding Name Name Date

the second line is particularly disorienting IMO because on two sides of the equality, `Date` denotes different things.

As far as i understand, in all contexts except those of `data` and `newtype` definitions, the equality sign in Haskell denotes the actual equality for all purposes: if a line

    foo x y = bar y x

is present in a program, `foo a b` and `bar b a` can be used more or less interchangeably elsewhere in the program.  Similarly, if the line

    type Name = String

is present, `Name` can be used as `String`.  Clearly, the equality in

    data Date = Date Int Int Int

does not have such property.

I think that if `::=` was used instead of `=` in `data` and `newtype` definitions, this would suggest  to a newcomer that the syntax of the two sides might be different, and would helpfully remind of the Backus–Naur Form for syntax rules.  I think that a newcomer to Haskell, like myself, would have had a better chance of guessing the meaning of

    type Name = String
    data Date ::= Date Int Int Int
    data Anniversary ::= Birthday Name Date | Wedding Name Name Date

IMO this would make the program easier to read in general and the difference between `type` and `newtype` more clear.  Maybe the can even make the use of keywords redundant, by allowing to write simply

    Name = String
    Date ::= Date Int Int Int
    Anniversary ::= Birthday Name Date | Wedding Name Name Date

What do you think?

Alexey.