相关疑难解决方法(0)

如预期的那样，可以正常工作：

valFrac :: Fractional a => a
valFrac = undefined

fNum :: Num a => a -> a
fNum a = undefined

resFrac :: Fractional a => a
resFrac = fNum valFrac -- Works as expected because every
                       -- Fractional is also a Num.
                       -- So as expected, we can pass
                       -- a Fractional argument into
                       -- a Num parameter.

另一方面，以下方法也适用。我不明白为什么。

fFrac :: Fractional a => a -> a
fFrac a = undefined

valNum :: Num a => a
valNum …