f# - this construct causes code to be less generic... when applied to phantom type -

i trying implement dsl in f# small language. unfortunately compiler stops me dead in tracks when attempt constrain nodes hold additional type information (via phantom types).

the following lines of code illustrate issue:

type expr<'a> = | int of int | eq  of expr<int> * expr<int> | not of expr<bool>  let rec to_string (expr: expr<'a>) =    match expr   | int(n)   -> string n   | eq(x, y) -> sprintf "%s == %s" (to_string x) (to_string y)   | not(b)   -> sprintf "!%s" (to_string b) 

according compiler construct to_string x issues following warning:

construct causes code less generic indicated type annotations. type variable 'a has been constrained type 'int'.

following that, on next line, construct to_string b issues error:

type mismatch. expecting expr<int> given expr<bool> type int not match type bool

i can't seem find way circumvent behavior, , in fact can't find cause code being less generic had expected. if possible prefer solution doesn't entirely abandon phantom types.

am doing fundamentally wrong? compiler bug?

you need make to_string function generic:

let rec to_string<'a> (expr:expr<'a>) : string =    match expr   | int(n)   -> string n   | eq(x, y) -> sprintf "%s == %s" (to_string x) (to_string y)   | not(b)   -> sprintf "!%s" (to_string b) 

the type parameter inferred when call function

not (eq (int 1, int 2)) |> to_string