Question

我试图学习Haskell，但是我被卡在数字转换上，有人可以解释为什么Haskell编译器会对这段代码感到生气：

phimagic :: Int -> [Int]
phimagic x = x : (phimagic (round (x * 4.236068)))

它会输出错误消息：

problem2.hs:25:33:
    No instance for (RealFrac Int) arising from a use of `round'
    Possible fix: add an instance declaration for (RealFrac Int)
    In the first argument of `phimagic', namely
      `(round (x * 4.236068))'
    In the second argument of `(:)', namely
      `(phimagic (round (x * 4.236068)))'
    In the expression: x : (phimagic (round (x * 4.236068)))


problem2.hs:25:44:
    No instance for (Fractional Int)
      arising from the literal `4.236068'
    Possible fix: add an instance declaration for (Fractional Int)
    In the second argument of `(*)', namely `4.236068'
    In the first argument of `round', namely `(x * 4.236068)'
    In the first argument of `phimagic', namely
      `(round (x * 4.236068))'

我已经在方法签名上尝试了许多组合（添加了Integral，Fractional，Double等）。有些东西告诉我，文字4.236068与问题有关，但无法弄明白。

Answer 1

Haskell不会自动为您投放内容，因此x * y仅在x和y具有相同类型时才有效（您无法将Int乘以{{ 1}}，例如）。

Double

注意我们可以使用Prelude函数phimagic :: Int -> [Int] phimagic x = x : phimagic (round (fromIntegral x * 4.236068))更自然地表达iterate：

phimagic

将小数值舍入为Int

1 个答案: