Long type error for Weights of large type

[sjakobi]

I'm trying to use generic-random for the Arbitrary instance of a type with currently 67 constructors. I've defined Weights based on the old definitions use of frequency. I'm fairly confident that I'm covering all the constructors, but GHC doesn't seem to like it and I don't understand what it's trying to say. Any ideas?

BTW, all was fine and dandy (type-wise) while I was initially defining the weights as

(7 :: W "Const") % (1 :: W "Var") % … % undefined

up to where I had defined about 10 weights.

$ cabal test -w ghc-8.6.5 dhall:tasty
Build profile: -w ghc-8.6.5 -O1
In order, the following will be built (use -v for more details):
 - dhall-1.25.0 (test:tasty) (file tests/Dhall/Test/QuickCheck.hs changed)
Preprocessing test suite 'tasty' for dhall-1.25.0..
Building test suite 'tasty' for dhall-1.25.0..
[13 of 15] Compiling Dhall.Test.QuickCheck ( tests/Dhall/Test/QuickCheck.hs, /home/simon/src/dhall-haskell/dist-newstyle/build/x86_64-linux/ghc-8.6.5/dhall-1.25.0/t/tasty/build/tasty/tasty-tmp/Dhall/Test/QuickCheck.o )

tests/Dhall/Test/QuickCheck.hs:249:19: error:
    • Couldn't match type ‘(Generic.Random.Internal.Generic.L
                              "Equivalent",
                            Int,
                            (Generic.Random.Internal.Generic.L "Note"
                             Generic.Random.Internal.Generic.:| (Generic.Random.Internal.Generic.L
                                                                   "ImportAlt"
                                                                 Generic.Random.Internal.Generic.:| Generic.Random.Internal.Generic.L
                                                                                                      "Embed"),
                             Int, ()))’
                     with ‘()’
      Expected type: Generic.Random.Internal.Generic.Prec'
                       ((Generic.Random.Internal.Generic.L "Assert"
                         Generic.Random.Internal.Generic.:| Generic.Random.Internal.Generic.L
                                                              "Equivalent")
                        Generic.Random.Internal.Generic.:| (Generic.Random.Internal.Generic.L
                                                              "Note"
                                                            Generic.Random.Internal.Generic.:| (Generic.Random.Internal.Generic.L
                                                                                                  "ImportAlt"
                                                                                                Generic.Random.Internal.Generic.:| Generic.Random.Internal.Generic.L
                                                                                                                                     "Embed")),
                        Int, ())
        Actual type: ()
    • In the second argument of ‘(%)’, namely ‘()’
      In the second argument of ‘(%)’, namely ‘(0 :: W "Note") % ()’
      In the second argument of ‘(%)’, namely
        ‘(7 :: W "ImportAlt") % (0 :: W "Note") % ()’
    |
249 |                 % ()
    |                   ^^

[Lysxia]

Thanks for opening this issue! The error messages could use some improvement. :)

That list is missing 4 constructors: IntegerToDouble, TextShow, Some, None. There are also some constructors in the wrong order (they must be in the same order as in the type definition): NaturaSubtract, Note.

[sjakobi]

Thanks a lot for figuring that out! I'm quite surprised that there still were missing constructors – it really demonstrates how easily a traditionally defined Arbitrary instance can get out of sync!

Regarding the order of constructors, I had somehow got the false impression, that I wouldn't need to stick to it if I use the W "X" annotations – not sure whether that should be pointed out in the docs.