Use of `inline` in F#

Question

The inline keyword in F# seems to me to have a somewhat different purpose than what I\'m used to in e.g. C. For example, it seems to affect a function\'s type (

Answer 1

When should I be using inline functions?

The most valuable application of the inline keyword in practice is inlining higher-order functions to the call site where their function arguments are also inlined in order to produce a singly fully-optimized piece of code.

For example, the inline in the following fold function makes it 5× faster:

  let inline fold f a (xs: _ []) =
     let mutable a = a
     for i=0 to xs.Length-1 do
        a <- f a xs.[i]
     a

Note that this bears little resemblance to what inline does in most other languages. You can achieve a similar effect using template metaprogramming in C++ but F# can also inline between compiled assemblies because inline is conveyed via .NET metadata.