How can i correct this error in functionnal programming?

Question

on the 44th line it gives me a type error Type automate is not compatible with type formatter im trying to draw an automaton using graphviz here is the code :

(*d'abord definissons le type automate*)

type automate = {
etat_initial : int;
ensemble_des_etats : int list;
alphabets : char list;
transitions :(int*char*int) list;
etats_finaux : int list
};;

(*prenons une variable a1 du type automate qu'on a definit precedemment 
  comme 
  exemple*)

let a1={
etat_initial=1;
ensemble_des_etats=[1;2];
alphabets=['a';'b'];
transitions=[(1,'b',2);(2,'c',3)];
etats_finaux=[2]
};;

let rec member a l =
match l with
| [] -> false
| x::rl -> x=a || member a rl;;


let fmt_transition auto fmt (inedge,by,outedge)=
if member outedge auto.etats_finaux=true then
 Format.fprintf fmt "@[node [shape = doublecircle]%d;@]" outedge;
 if inedge=auto.etat_initial then
 Format.fprintf fmt "@[node [shape = point]start;node [shape = circle];start 
 -> %d ;@]" inedge;
 Format.fprintf fmt "@[%d -> %d [label=\"%c\"];@]" inedge outedge by;;

let fmt_transitions auto fmt =
Format.fprintf fmt "@[<v 2>digraph output {@,%a@,@]}@,@."
(Format.pp_print_list (fmt_transition auto)) auto.transitions
;;

 let call_dot auto =
 let cmd = "dot -Tpng | display -" in
 let (sout, sin, serr) as channels =
 Unix.open_process_full cmd (Unix.environment ()) in
 let fmt = Format.formatter_of_out_channel sin in
 <b>Format.fprintf fmt "%a@." fmt_transitions auto;</b>
 channels

let cleanup channels =
(* missing: flush channels, empty buffers *)
 Unix.close_process_full channels;;

call_dot a1 ;;

Also related to the questions [How can I represent an automaton graphically from a list of int*char*int representing the transitions without using loops](http://stackoverflow.com/questions/41780982/how-can-i-represent-an-automaton-graphically-from-a-list-of-intcharint-represe) and [how to draw an automaton diagram?](http://stackoverflow.com/questions/41901071/how-to-draw-an-automaton-diagram). — lambda.xy.x, Feb 09 '17 at 22:45

score 1 · Answer 1 · answered Feb 09 '17 at 13:14

You need to be careful when you use %a. According to the OCaml documentation:

a: user-defined printer. Take two arguments and apply the first one to outchan (the current output channel) and to the second argument. The first argument must therefore have type out_channel -> 'b -> unit ...

The first argument argument of your fmt_transitions auto fmt function has to be a formatter, so just switch the auto and fmt arguments and it should be OK.

let fmt_transitions auto fmt = ...

let fmt_transitions fmt auto = ...

How can i correct this error in functionnal programming?

1 Answers1