Why can't Console.WriteLine determine my type? in F# -

here's code:

open system  let places = [ ("grandchester", 552);                ("cambridge", 117900);                ("prague", 1188126); ]  let statusbypopulation = function                             | n when n > 1000000 -> "city"                             | n when n > 50000   -> "town"                             | _                  -> "village"  system.console.writeline ( places |> list.map (fun (_, population) -> statusbypopulation population))   let print x =      console.writeline (list.map (fun (_, population) -> statusbypopulation population) x) // i'm trying  let (x:(string * int) list) =      list.map (fun (_, population) -> statusbypopulation population) x; // checking kinf of type returns  let print: (string * int) list -> unit =      console.writeline << list.map (fun (_, population) -> statusbypopulation population) // i'm not allowed  system.console.readkey () |> ignore 

i wanted familiar way function composition operator worked, reason f# can't find best possible overload function...

in example explicitly state parameter, sets type val print : x:('a * int) list -> unit, explicitly set type in function composition operator << hoping i'd correct result... didn't...

i made function something explicitly declared type parameter, see it'd return... returns this: val : x:(string * int) list -> string list

so returns type... list of strings, know console.writeline capable of printing... why tell me can't determine overload?

the type inference in f# works left right - means compiler uses information available earlier in program determine types of expressions later in program (this slight simplification, general idea).

so in code, when write:

console.writeline << list.map (fun (_, population) -> statusbypopulation population) 

.. compiler not propagate information type of function input through list.map call writeline call. explains why forward chaining , composition more useful in f#. following works:

list.map (fun (_, population) -> statusbypopulation population) >> console.writeline 

to original code working, provide minimal amount of information needed determine right writeline overload 1 taking object. if tell compiler needs take list of something, can choose right overload:

(console.writeline:list<_> -> unit) << list.map (fun (_, population) ->      statusbypopulation population)