# case statement and guarded equations

4 messages
Open this post in threaded view
|
Report Content as Inappropriate

## case statement and guarded equations

 In the case statement for the half function I think  variable m is bound to the value of (snd (half (n - 1)).  With this assumption, I have have written a guarded version called half1. Is it always possible to write case statements as a set of conditional equations? My intuition is that this should be possible and that the guarded version reduces pattern matching? Thanks,Patdata EvenOdd = Even | Odd deriving (Show,Eq)half 0 = (Even,0)half n = case half (n-1) of (Even,m) -> (Odd,m)                            (Odd,m) -> (Even,m+1) half1 0  =  (Even , 0) half1 n  |  (fst  (half (n - 1)) == Even)  = (Odd , (snd (half (n - 1))))half1 n  |  (fst  (half (n - 1)) == Odd)   = (Even , ((snd (half (n - 1))) + 1)) This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ieIs ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ieTá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman_______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
Open this post in threaded view
|
Report Content as Inappropriate

## Re: case statement and guarded equations

 Quoting the report[1], "A boolean guard, g, is semantically equivalent to the pattern guard True <- g," which means the answer is "Yes". A boolean guard is equivalent to a pattern match. A predicate involving ==, however, introduces an Eq constraint that would not be required by pattern matching. For a properly equivalent guard, you need to write your predicates using pattern matchingisEven Even = TrueisEven _ = Falseto avoid the spurious Eq constraint. _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
Open this post in threaded view
|
Report Content as Inappropriate

## Re: case statement and guarded equations

 I'll also mention that GHC's exhaustiveness checker will mark the pattern match as exhaustive (since EvenOdd must be either Even or Odd and both cases are given) but warn about the two guards since it doesn't know that they form a dichotomy. You can use `otherwise`, which is just another name for True, to convince GHC that your guards are exhaustive.On Fri, Jul 7, 2017 at 3:38 PM Rein Henrichs <[hidden email]> wrote:Quoting the report[1], "A boolean guard, g, is semantically equivalent to the pattern guard True <- g," which means the answer is "Yes". A boolean guard is equivalent to a pattern match. A predicate involving ==, however, introduces an Eq constraint that would not be required by pattern matching. For a properly equivalent guard, you need to write your predicates using pattern matchingisEven Even = TrueisEven _ = Falseto avoid the spurious Eq constraint. _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners