Haskell 元组大小限制

问题描述

为什么我不能在 Haskell 中构造大元组?为什么有元组大小限制?

Why I can't construct large tuples in Haskell? Why there's a tuple size limit?

Prelude> (1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1)

<interactive>:1:0:
    No instance for (Show
                       (t,
                        t1,
                        t2,
                        ...
                        t23))
      arising from a use of `print' at <interactive>:1:0-48
    Possible fix:
      add an instance declaration for
      (Show
         (t,
          t1,
          t2,
          ...
          t23))
    In a stmt of a 'do' expression: print it

推荐答案

元组可以是任意长度*，但 Show 以及 Eq、Ord、Read、Bounded 等只能实例化最多 15 个元组.来自 Haskell 98 报告 §6.1.4:

Tuples can be of arbitrary length*, but Show, as well as Eq, Ord, Read, Bounded, etc are only instantiated up to 15-tuple. From the Haskell 98 report §6.1.4:

元组的大小没有上限，但是一些 Haskell 实现可能会限制元组的大小，并限制与较大元组关联的实例.但是，每个 Haskell 实现都必须支持最大 15 的元组，以及 Eq、Ord、Bounded、Read 和 Show 的实例. Prelude 和库定义了元组函数，例如 zip 的元组最大为大小为 7.

正如其他人所说，如果你需要一个 24 元组，你应该使用更好的数据结构.

As others have said, if you need a 24-tuple, you should use a better data structure.

* 自 GHC 6.12.2 起，元组的最大大小为 62:

* as of GHC 6.12.2, the maximum size of a tuple is 62:

Prelude> :t (1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8)

<interactive>:1:0:
    A 64-tuple is too large for GHC
      (max size is 62)
      Workaround: use nested tuples or define a data type

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