Instances with GHC.TypeLits.Nat/Symbol should be possible without FlexibleInstances.

Yes I know, 0 and "B" are not technically type constructors, but it's as close as you can get for the kinds Nat and Symbol.

Test cases:

{-# LANGUAGE DataKinds, KindSignatures #-}

import GHC.TypeLits (Nat, Symbol)

class A (n::Nat)
instance A 0

class B (s::Symbol)
instance B "B"

Result:

A.hs:6:10: error:
    • Illegal instance declaration for ‘A 0’
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘A 0’

A.hs:9:10: error:
    • Illegal instance declaration for ‘B "B"’
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are *distinct type variables*,
         and each type variable appears at most once in the instance head.
         Use FlexibleInstances if you want to disable this.)
    • In the instance declaration for ‘B "B"’

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 3

added Tbug Trac import labels

This should be straightforward to fix: it would just require some tweaking of the tcInstHeadTyAppAllTyVars function to allow something headed by a LitTy.

added newcomer label

Um, is there anybody using fancy type stuff like TypeLits, but not needing FlexibleInstances? I know `-fglasgow-exts` is deprecated these days, but I always switch on those extensions anyway.

I'd like to try solving this one

Do let us know if you get stuck, qrilka.

Trac metadata

Trac field	Value
Differential revisions	- → D4823
CC	- → qrilka@gmail.com

bgamari, should I do something else about my patch?

mentioned in commit 42f3b53b

closed

changed milestone to %8.6.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added Plow label