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

Clinton Mead clintonmead at
Wed Feb 23 02:08:20 UTC 2022

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

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

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.

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