Question

在第44行它给了我一个类型错误类型自动化与类型formatter不兼容即时尝试使用graphviz绘制自动机这是代码：

(*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 ;;

Answer 1

使用%a时需要小心。根据OCaml文档：

a：用户定义的打印机。取两个参数并将第一个参数应用于 outchan （当前输出通道）和第二个参数。 因此第一个参数必须具有类型out_channel - ＆gt; 'b - ＆gt;单位 ...

fmt_transitions auto fmt函数的第一个参数参数必须是格式化程序，所以只需切换auto和fmt参数即可。

let fmt_transitions auto fmt = ...

let fmt_transitions fmt auto = ...

如何在功能编程中纠正此错误？

1 个答案: