I'm pretty sure I understand this. I briefly looked into OCaml, so this
is familiar. Are the alternates tried in order or is there some notion
of "best match" like with template specialzations?
There is a funky algorithm to do this quite fast
If you "forgot" a case that make the function not working, ML will
complain and in some case give you the missing case.

How in the world is that computable in general? Is match...with
restricted to list-like thingies?
Nope. The typer "tries" to help you by giving you an example of missing stuff.
If it can't he just say "missing match cases dude"

Cool. So you're using this to validate your semantic actions?
to debug them in a friendly environment ;)
Now, look at this again and replace let with template<class T> struct
len and match by template specialization. Bingo, you got the exact same
function turned into a meta-function.

I object! The len function computes the length of a *runtime* list.
Obviously with template specializations we're no longer dealing with
runtime data structures. I don't see how to the two relate.
Runtime values in ML <-> Compile-time value in C++ TMP
A function in ML operatirng on list of values becomes a meta-funtion in C++ operating on types list.
Both languages beign pure and functionnal helps the transition.

Just make a ML compound type like

type Type = f of Float | d of Double | i of Integer;

and let's have a polymorphic list of Type.

let l = len [ Float; Flaot; Double ];

see where we're going ?

Good luck!
The stuff is pretty much here. Need some sla... students to do the work ;)
In theory it should be possible to add a theorem prover to proto to
validate semantic actions, right? Not that C++ TMP is necessarily the
best tool for this.
I think it's better as an outside tools. However, a DSL for HOARE logic injection in C++ is another beast ;)

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