Thanks for the response. It appears the bug was already reported in a slightly different context. https://ghc.haskell.org/trac/ghc/ticket/11216 On Wed, Dec 21, 2016 at 6:24 AM, Richard Eisenberg wrote:

Sounds like a bug to me.... and one I may have run into at some point. Would you care to file a bug report?

Thanks! Richard

...

On Dec 20, 2016, at 3:53 AM, Chris Smith wrote:

Hey everyone,

Can someone explain to me by GHC 8 causes list comprehension bindings to desugar to use 'fail', when RebindableSyntax is enabled in GHC 8? Note that I do not have monad comprehensions enabled. If I define fail, everything works (apparently regardless of what I define fail to do?!?), but I'd like to understand why it's necessary.

Thanks, Chris _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

Thanks Richard, I will file the bug shortly. David, fail doesn't appear to be used at all, either for pattern match failure (which I initially expected), or for guards. Example here: https://code.world/#P2ZO5lhXKM5QGYh-vRd31Ug On Wed, Dec 21, 2016 at 12:56 PM, David Menendez wrote:

On Tue, Dec 20, 2016 at 3:53 AM, Chris Smith wrote:

...

Hey everyone,

Can someone explain to me by GHC 8 causes list comprehension bindings to desugar to use 'fail', when RebindableSyntax is enabled in GHC 8? Note that I do not have monad comprehensions enabled. If I define fail, everything works (apparently regardless of what I define fail to do?!?), but I'd like to understand why it's necessary.

Perhaps fail is used by the translation for guards. I don’t have a working GHC right now, but what happens if you define fail = error and evaluate [ x | x <- “a", False]?

-- Dave Menendez http://www.eyrie.org/~zednenem/