This doesn't buy you very much, unfortunately. (f $!) only forces its argument if the result is ever forced. So if the implementation of fmap doesn't force the function calls (and normally it won't, with no possible way to use the result for anything) then f x doesn't either; the arguments to those calls are not forced, and so the contents of the original structure remain unforced. For example: Prelude> let f x = (f $!) x() :: Functor f => (a -> b) -> f a -> f b Prelude> let z = const True [undefined]z :: [Bool] Prelude> case z of (_:_) -> "matched""matched"it :: String Prelude> z[*** Exception: Prelude.undefined The undefined bomb only goes off when I start to force the elements of z; prior to that no strictness has been gained, and the list contains a thunk as normal. In particular, this version of wouldn't help with the lazy IO problem described in that stackoverflow question Christopher linked earlier. -- Ben ----- Original Message ----- From: "Will Yager" To:"Christopher Allen" Cc:"haskell-cafe" Sent:Mon, 10 Aug 2015 14:30:49 -0700 Subject:Re: [Haskell-cafe] Constraints on vs What about f x = (f $!) x ? --Will On Aug 10, 2015, at 14:20, Christopher Allen wrote: http://stackoverflow.com/questions/9423622/strict-fmap-using-only-functor-no... [2] seems to cover it. On Mon, Aug 10, 2015 at 4:17 PM, Alexey Egorov wrote: Hello haskell ers, I wonder why and have different typeclass constraints? () :: Functor f => (a -> b) -> f a -> f b () :: Monad m => (a -> b) -> m a -> m b _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org [4] http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe [5] -- Chris Allen Currently working on http://haskellbook.com [6] _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org [7] http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe [8] Links: ------ [1] mailto:cma@bitemyapp.com [2] http://stackoverflow.com/questions/9423622/strict-fmap-using-only-functor-no... [3] mailto:electreg@list.ru [4] mailto:Haskell-Cafe@haskell.org [5] http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe [6] http://haskellbook.com [7] mailto:Haskell-Cafe@haskell.org [8] http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe