Subtle difference between standard instances

David Sankel camior at
Tue Oct 28 12:15:17 EDT 2008

We, at Anygma, noticed that a and b below have different behavior:

a, b :: (Int->Int) -> (Int,Int) -> (Int,Int)
a = fmap
b = second

See the blog post at

The standard instances for Functor, Monad, and Applicative for tuples is
strict. The Arrow on (->) when applied to tuples with first and second,
however is non-strict.

It is a subtle discrepancy, but it does make a difference for FRP when
deriving these standard instances. Any objections to making the Functor,
Monad, and Applicative instances for tuples non-strict like Arrows?

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