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February 2022

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Are constructors matched against using "switch" or chained if-else
by Clinton Mead 23 Feb '22

23 Feb '22

Hi All I'm developing an unbounded integer type, which I won't go into the details here but in some circumstances has better performance than the standard "Integer". Anyway, whilst there are complex cases, the most common case is a standard machine int multiplication. Hence I want the type to be optimised for that case. I'm going to have a few constructors, anyway, so I first considered something like this: `data MyInt = BasicZero | BasicPos Word | BasicNeg Word | ComplexPosA ... | ComplexNegA ... | ComplexPosB ... | ComplexNegB ...` I'd naturally make the "Word"s in "BasicPos" and "BasicNeg" strict/unpack, hopefully eliminating the indirection, or perhaps just making them primitive directly. This has 7 constructors, which quite nicely I believe fits into the three spare bits in a 64 bit pointer which GHC optimises I believe. However, this approach I thought of because I assumed that GHC would do a switch style statement on the constructors, so once I have more than one I might as well have as many as I want (well, up to 7, until I lose the optimisation). But if GHC compiles this to a "if ... else if..." chain, a better representation is the following: `data MyInt = BasicInt Int | ComplexPosA ... | ComplexNegA ... | ComplexPosB ... | ComplexNegB ...` That way I can match against BasicInt first, and knock that out of the way as my "hot" case. However, using "Int" instead of "Word" does make the logic a bit more complex, but it may be worth it if I'm reducing the number of patterns I have to check against for all arguments. I was just wondering if anyone could share some insight on what GHC does in these circumstances? For example, does the order I list my cases in a case statement matter if they're non-overlapping? Will GHC match them in the order I list, or will it just make them into a switch statement so it doesn't matter if I reorder them? I guess I could benchmark all this (and probably will) but some insights and general guidance would be good. Thanks, Clinton

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