Brian Hulley wrote:
Brian Hulley wrote:
Yitzchak Gale wrote:
I wrote:
Prelude> let f = undefined :: Int -> IO Int Prelude> f `seq` 42 *** Exception: Prelude.undefined Prelude> ((>>= f) . return) `seq` 42 42 The monad laws say that (>>= f) . return must be identical to f.
I thought it was:
return x >>= f = f x
so here the lhs is saturated, so will hit _|_ when the action is executed just as the rhs will.
Ooops! But that does not mean the equation holds because for example Prelude> (return 3 >>= f) `seq` 42 42 Prelude> (f 3) `seq` 42 *** Exception: Prelude.undefined In the lhs you only hit _|_ when the composite (>>=) action is actually being executed whereas in the rhs you hit _|_ when computing the function which will return the action to execute so there is difference. Brian. -- http://www.metamilk.com