F# would like to name the first inferred parameter 'a, then 'b, 'c and so on. If it’s in use, it will be suffixed with 0..n. Let’s look at an example:

let fn a b c d e f g h = () 
//a:'a -> b:'b -> c:'c -> d:'d -> e:'e -> f:'f -> g:'g -> h:'h -> unit

So if you constrain a to something, it’ll start naming from the next unconstrained parameter - b onwards.

let fn (a : 'fst) b c d e f g h = ()
//a:'fst -> b:'a -> c:'b -> d:'c -> e:'d -> f:'e -> g:'f -> h:'g -> unit

When you constrain b, it will have the same sequence, but skip b, because we named it.

let fn a (b : 'snd) c d e f g h = ()
//a:'a -> b:'snd -> c:'b -> d:'c -> e:'d -> f:'e -> g:'f -> h:'g -> unit

So what happens when you manually constrain with a name in the series? It’s about to go from 'a to 'b, sees 'b already in use, so uses a 'b0 instead.

let fn a (b : 'b) c d e f g h = ()
//a:'a -> b:'b -> c:'b0 -> d:'c -> e:'d -> f:'e -> g:'f -> h:'g -> unit

This sequence will continue - if 'b0 is in use, 'b1 is chosen.

let fn a (b : 'b) (c: 'b0) d e f g h = ()
//a:'a -> b:'b -> c:'b0 -> d:'b1 -> e:'c -> f:'d -> g:'e -> h:'f -> unit

Last modified on 2019-01-31