Are constructors matched against using "switch" or chained if-else

David Feuer david.feuer at
Wed Feb 23 02:34:13 UTC 2022

I can answer one of your questions for sure: the order of your case
branches doesn't matter at all. However, the order of the data constructors
in the type declaration does matter. Put your most likely one first.

On Tue, Feb 22, 2022, 9:09 PM Clinton Mead <clintonmead at> wrote:

> 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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