There are no guarantees about in what order these things will be evaluated. The compiler is well within its rights to evaluate the expressions in any order, or more than once even (though IIRC ghc never does the latter). The left-to-right ordering holds for && because the Haskell report specifically defines[1] it as: True && x = x False && _ = False In this case the compiler can't in general evaluate the RHS first, because if the LHS is False and the RHS is bottom, this would be incorrect. But this is due to the semantics of &&, and doesn't hold in general. -Ian [1]: https://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 Quoting erik (2018-12-23 15:19:04)
Tom, Why does "b" evaluate before "a" in your example? I would have thought left-hand and then right-hand if needed. I'm on my phone but I'd like to try the same example with booleans and `&&` instead of `+`. Erik On Sat, Dec 22, 2018, 11:40 PM Tom Ellis <[1]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk wrote:
On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien Mattei wrote: > i have inserted trace statement that output variable > ... i have strange behavior of output: Let's take a simpler example.� Do you understand why the trace statments from this small program appear in the order that they do?� (And for what it's worth I really think you'll be better off writing programs using do notation). % cat test.hs import Debug.Trace result = � let a = trace "evaluating a" 2 � � � b = trace "evaluating b" 10 � � � c = trace "evaluating c" (a + b) � in c ~% ghci -e result test.hs evaluating c evaluating b evaluating a 12 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: [2]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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