Adding to a set in Ocaml -

i have fundamental issue in ocaml has been bugging me long time now. have pattern match compares input user , prints out concatenated string. works fine need add concatenated string set within pattern match. each time try error:

this expression has type ss.t = set.make(string).t    expression expected of type string 

i new ocaml , has taken many days , have not been able figure out. code pattern matching below:

type t = 0 | pproc of string | procdef of t * t  module ss = set.make(string) let set2= ss.empty  let concattoset s =     list.fold_right ss.add [s] set2  let rec poc p = match p   | 0 -> concattoset "0"   | pproc x -> concattoset x   | procdef (p1, p2) -> concattoset((poc p1)^"("^(poc p2)^")") 

the error have :

procdef (p1, p2) -> concattoset((poc p1)^"("^(poc p2)^")");;                                 ^^^^^^^^ error: expression has type ss.t = set.make(string).t        expression expected of type string 

i edited question available once peer reviewed. nevertheless, let's answer question (by way, don't "thanks!" , try give example can reproduce error.)

so, code looks :

type t = 0 | pproc of string | procdef of t * t  module ss = set.make(string) let set2= ss.empty  let concattoset s =     list.fold_right ss.add [s] set2  let rec poc p = match p   | 0 -> concattoset "0"   | pproc x -> concattoset x   | procdef (p1, p2) -> concattoset((poc p1)^"("^(poc p2)^")") 

and error have :

procdef (p1, p2) -> concattoset((poc p1)^"("^(poc p2)^")");;                                 ^^^^^^^^ error: expression has type ss.t = set.make(string).t        expression expected of type string 

first of all, set immutable ss.add return new set, not unit.

according this, type of concattoset ss.elt -> ss.t return type of poc same type of concattoset "0").

that's because when compiler tries type function pattern matching infers type according returned each case of pattern matching :

let rec poc p = match p    | 0 -> concattoset "0"    | ... 

here, p of type t because it's matched zero , concattoset of type string -> ss.t concattoset "0" of type ss.t poc of type t (*the type of p*) -> ss.t (the type returned concattoset "0".

when call concattoset ((poc p1) ^ poc p2)), poc p1 expected string (concattoset : string -> ss.t) has typess.t(the return type ofpoc p1`) , that's compiler told you.

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an (ugly) idea of code write :

type t = 0 | pproc of string | procdef of t * t  module ss = set.make(string)  let concattoset ?s1 ?sep1 ?sep2 s2 set =     let str = printf.sprintf "%s%s%s%s"         (match s1 none -> "" | s -> s)        (match sep1 none -> "" | s -> s)        s2        (match sep2 none -> "" | s -> s)     in     ss.add str set;;  let rec poc p set = match p   | 0 -> concattoset "0" set   | pproc x -> concattoset x set   | procdef (p1, p2) -> concattoset ~s1:(poc p1) ~sep1:"(" ~sep2:")" (poc p2) set 

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but that's bit awkward , if types has multiple constructors rapidly hard write concattoset.

what following (added timer more complex constructor):

let rec pp fmt = function   | 0 -> format.fprintf fmt "0"   | pproc x -> format.fprintf fmt "%s" x   | procdef (p1, p2) -> format.fprintf fmt "%a(%a)" pp p1 pp p2   | timer(t,chan, var, p1, p2) -> format.fprintf fmt "timer%s(%s(%s).%a,%a)" t chan var pp p1 pp p2;;   let concattoset s set = ss.add s set;;  let poc p set = concattoset (format.asprintf "%a" pp p) set;; 

so, pp takes formatter (which can seen output buffer) prints in constructors necessary recursive calls. concattoset simple ss.add , poc takes parameter of type t , set , calls pp formatter being string , gives new string concattoset want have.

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about %a :

it may seem bit strange pp expecting 2 arguments give 1 when call, example format.asprintf "%a" pp p. well, actually, i'm not applying pp p, here, i'm giving 2 arguments printer said in documentation :

a: user-defined printer. take 2 arguments , apply first 1 outchan (the current output channel) , second argument. first argument must therefore have type out_channel -> 'b -> unit , second 'b. output produced function inserted in output of fprintf @ current point.

that's why writing format.asprintf "%a" (pp p) wouldn't accepted compiler.