Comment by troad

1 day ago

String literal typing appears to be a common feature of type systems bolted onto dynamic languages:

    # Python
    MyStringBool = Literal("Yes") | Literal("No")

    // TypeScript
    type MyStringBool = "Yes" | "No"

I assume it exists to compensate for the previous lack of typing, and consequent likelihood of ersatz typing via strings.

It would seem pretty unnecessary in Haskell, where you can just define whatever types you want without involving strings at all:

    data MyBool = Yes | No

Of course you'd need a trivial parser, though this is probably a good idea for any string type:

    parseMyBool :: String -> MyBool 
    parseMyBool "Yes" = Yes
    parseMyBool "No" = No
    parseMyBool _ = error "..."

Interestingly, dynamic languages which make use of symbols (Ruby, Elixir, Common Lisp) probably fall closer to Haskell than Python or TS. Elixir example:

    @type my_bool() :: :yes | :no

    @spec parse_my_bool(String.t()) :: my_bool()
    def parse_my_bool("Yes"), do: :yes
    def parse_my_bool("No"), do: :no
    def parse_my_bool(_), do: throw("...")

Where :yes and :no are memory-efficient symbols, not strings.

12 comments

troad

jaen 14 hours ago

String literals are structural types which are way more expressive than regular (Haskell) ADTs, which are nominal types.

In TS in particular, in combination with other features (mapped types), they are equivalent to row polymorphism + whatever Haskell/GHC features enable type families to specialize on constant literal arguments (or you can use atomic types, but that's not structural / open-world)... so pretty advanced.

This is valid TS/Python:

    type ABC = "A" |"B" | "C"
    type AB = "A" | "B"
    const x: AB = "A";
    const y: ABC = x;

The equivalent Haskell requires using several extensions.

troad 13 hours ago
I know. I literally gave the example of a Python Literal in the post you're replying to. TS too. :)
My overall point is that Haskell's type system is sufficiently expressive (you may not have "A" | "B" | "C", but you do have A | B | C) that there's no obvious remaining use case for string literals, unless you're thinking of typing input by way of expected literals instead of actually parsing it, which is... a choice. :P
- jaen 12 hours ago
  
  By Haskell's type system do you mean with all the GHC extensions?
  Because TypeScript has structural sub-typing, while standard Haskell (eg. `A | B | C`) has neither subtyping nor structural typing, which both are very useful features for safe "integration/glue" type of programs.
  (String) literals form a fundamental part of the TS "row polymorphism" (record types) and eg. tuple union type implementation.
  You can type a non-empty array that starts with zero...
  type Arr = [0, ...number[]]; const a: Arr = [0, 1, 2, 3, 4]
  Now try in Haskell.
  
  6 replies →

nesarkvechnep 16 hours ago

Thank you for the lecture but I didn’t mean that. I meant `Either String String` is possible in Haskell and not in C# because… C# is strongly-typed.

troad 16 hours ago

You're welcome! Knowing is half the battle.
That Haskell snippet is just syntax sugar for Left(string) | Right(string), which is trivial in any language with unions.
Not clear why it would be an improvement over just naming the alternatives something meaningful, but if you're wedded to Left and Right, go for it.