haskell - 为什么 GHC 打印 15 元组而不打印 16 元组？

Question

为什么这行得通

print (True, True, True, True, True, True, True, True, True, True, True, True, True, True, True)

虽然这不是

print (True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True)

Answer 1

因为有Show例如 15 元组:

Prelude> :i (,,,,,,,,,,,,,,)
data (,,,,,,,,,,,,,,) a b c d e f g h i j k l m n o
  = (,,,,,,,,,,,,,,) a b c d e f g h i j k l m n o
    -- Defined in `GHC.Tuple'
<<skip>>
instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g,
          Read h, Read i, Read j, Read k, Read l, Read m, Read n, Read o) =>
         Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)
  -- Defined in `GHC.Read'
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i, Show j, Show k, Show l, Show m, Show n, Show o) =>
         Show (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)
  -- Defined in `GHC.Show'

并且没有 16 元组:

Prelude> :i (,,,,,,,,,,,,,,,)
data (,,,,,,,,,,,,,,,) a b c d e f g h i j k l m n o p
  = (,,,,,,,,,,,,,,,) a b c d e f g h i j k l m n o p
    -- Defined in `GHC.Tuple'

见 docs

AFAIK 实例是在 ghc 内部库中的某处手写的，不太可能有人需要显示 16 元组。