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?