New syntax for lists and tuples

Originally posted on wipple.org.

For a long time, Wipple has used the list syntax to construct a list and the , syntax to construct a tuple. Today, this changes — the , syntax is now used for lists!

numbers : 1 , 2 , 3
numbers . each show

1
2
3

I decided to make this change for two reasons. First, lists are used much more often than tuples, so it makes sense to give list syntax priority. Second, Wipple parses syntax rules before resolving variables, so having a syntax rule named list means that you can’t declare a variable named list as well. The standard library worked around this by using names like l or input, but now you can just use the obvious variable name list.

If you provide elements of different types, you still get a nice error message:

my-list : 1 , "2"

error:
  ┌─ test.wpl:1:15
  │
1 │ my-list : 1 , "2"
  │               ^^^
  │               │
  │               expected `Number`, but found `Text`
  │               this element must have the same type as the other elements
  │
  = for more information, see https://wipple.dev/playground/?lesson=errors/mismatched-types

To create an empty list, use the , operator by itself (or the Default implementation defined below):

instance (Default (List _)) : (,)

And to create a list with a single element:

just-one-number :: List Number
just-one-number : 1 ,

Trailing commas are allowed, so you can easily add a new item to a large list:

constants : (
    1.41 ,
    1.62 ,
    2.72 ,
    3.14 ,
    6.28 ,
)

The , syntax for lists is defined in Wipple, too, meaning Wipple now supports variadic operators!

[operator Variadic-Precedence]
, : syntax {
    , ...elements -> ...
}

And finally, to create a tuple, you now separate each element with a semicolon (;):

my-tuple :: Number ; Text ; Boolean
my-tuple : 1 ; "a" ; True

first ; second ; third : my-tuple
show first
show second
show third

1
a
True

These changes are live on the Wipple Playground, and the lessons have been updated to use the new syntax.