[PureScript] Add module Undefined as dependency

[DonaldKellett]

Unlike Haskell, PureScript does not have a polymorphic undefined value by default and enforces totality so it may be impossible to provide an initial solution that compiles in certain cases without resorting to ugly hacks such as Unsafe.Coerce.unsafeCoerce Prelude.unit. However, a handy module Undefined (source) with a value undefined has been made available in PureScript since April 2018 which does not appear to be available on Codewars. Please consider adding it as a dependency, thanks 😄

[kazk]

We use psc-package so a package needs to be the one in package-sets. purescript-undefined doesn't seem to be.

Is this package commonly used? It also needs to be compatible with 0.12 (May 2018).

See these issues and documentation

• https://github.com/purescript/documentation/blob/master/language/Differences-from-Haskell.md#error-and-undefined
• add undefined to Prelude purescript/purescript-prelude#44
• Typed holes purescript/purescript#1283

[kazk]

Maybe updating to v0.12.2 will help. It supports named type wildcards ?hole.

[kazk]

I might be wrong, but the package you're suggesting won't behave like Haskell's undefined. I think it will just give a runtime type error that it's not a function. And it's confusing because it'll be in JavaScript syntax.

---

I think ?undefined (or name that's more appropriate) is good enough.

PureScript

countSheep :: Int -> String
countSheep = ?undefined

won't compile and shows

Error found:
in module CountSheep
at src/CountSheep.purs line 9, column 14 - line 9, column 14

  Hole 'undefined' has the inferred type

    Int -> String

  You could substitute the hole with one of these values:

    CountSheep.countSheep  :: Int -> String


in value declaration countSheep

(I can see this can be annoying for more complex kata though)

Haskell

countSheep :: Int -> String
countSheep = undefined

will compile, but the error is

ErrorCall (Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
  undefined, called at src/CountSheep.hs:4:14 in main:CountSheep)

[DonaldKellett]

Good point - typed holes should provide a clearer error message than TypeError: CountSheep.countSheep is not a function (which is what happens when undefined = unsafeCoerce unit is used) which is arguably even less readable than a compilation error message. Naming it ?todo should get the message across to beginners that the code would compile once the hole is replaced with a sensible solution implementation.