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Refactor (~) to reduce the superclass stack

    • Closed Issue created by Simon Peyton Jones @simonpj

    Currently (see Note [The equality types story] in TysPrim) we have

      -- Hetereogeneous equality
  class a ~# b => a ~~ b
  instance a ~# b => a ~~ b

  -- Homogeneous equality
  class a ~~ b => (a :: k) ~ (b :: k)
  instance a ~~ b => a ~ b

    Note that (~#) is a superclass of (~~), and (~~) is a superclass of (~). This means that in the common case of using (~) we need two superclass selections to get to the (~#) we want.

    Nothing really wrong with that, but it bloats programs, and is confusing to read when debugging. I propose to change this to

      -- Homogeneous equality
  class a ~# b => (a :: k) ~ (b :: k)
  instance a ~# b => a ~ b

    That is, implement (~) in precisely the same way as (~~).

    This makes (~) a tiny bit more baked-in to the compiler, but in exchange it behaves in the same way as (~~), instead of behaving in a slightly different way.

    There should be no observable effect for users.

    Trac metadata
    Trac field Value
    Version 8.4.3
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
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    Blocking
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    Operating system
    Architecture
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