How does pattern matching work behind the scenes in F#?

Question

I am completely new to F# (and functional programming in general) but I see pattern matching used everywhere in sample code. I am wondering for example how pattern matching actually works? For example, I imagine it working the same as a for loop in other languages and checking for matches on each item in a collection. This is probably far from correct, how does it actually work behind the scenes?

Answer 1

How does pattern matching actually work? The same as a for loop?

It is about as far from a for loop as you could imagine: instead of looping, a pattern match is compiled to an efficient automaton. There are two styles of automaton, which I call the "decision tree" and the "French style." Each style offers different advantages: the decision tree inspects the minimum number of values needed to make a decision, but a naive implementation may require exponential code space in the worst case. The French style offers a different time-space tradeoff, with good but not optimal guarantees for both.

But the absolutely definitive work on this problem is Luc Maranget's excellent paper "Compiling Pattern Matching to Good Decisions Trees from the 2008 ML Workshop. Luc's paper basically shows how to get the best of both worlds. If you want a treatment that may be slightly more accessible to the amateur, I humbly recommend my own offering When Do Match-Compilation Heuristics Matter?

Writing a pattern-match compiler is easy and fun!