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  1. type Context = i32;
  2.  
  3. fn run_sync_1(ctx: Context, next: &Fn(Context) -> Context) -> Context {
  4.     let ctx_new = ctx + 1;
  5.    
  6.     next(ctx_new)
  7. }
  8.  
  9. fn run_sync_2(ctx: Context, next: &Fn(Context) -> Context) -> Context {
  10.     let ctx_new = ctx * 2;
  11.    
  12.     next(ctx_new)
  13. }
  14.  
  15. fn run_sync_3(ctx: Context, _next: &Fn(Context) -> Context) -> Context {
  16.     ctx - 1
  17. }
  18.  
  19. type ChainType<T> = fn(T, &Fn(T) -> T) -> T;
  20.  
  21. fn main() {
  22.     // Type of impl Fn(Context, impl Fn(Context) -> Context) -> Context
  23.  
  24.     // Sets to 1
  25.     fn chain_a(context: Context, end: &Fn(Context) -> Context) -> Context {
  26.         run_sync_1(context, &|context| {
  27.             run_sync_2(context, &|context| {
  28.                 run_sync_3(context, end)
  29.             })
  30.         })
  31.     };
  32.     let chain_1: ChainType<Context> = chain_a;
  33.    
  34.     // Adds 3
  35.     fn chain_b(context: Context, end: &Fn(Context) -> Context) -> Context {
  36.         run_sync_1(context, &|context| {
  37.             run_sync_1(context, &|context| {
  38.                 run_sync_1(context, end)
  39.             })
  40.         })
  41.     };
  42.     let chain_2: ChainType<Context> = chain_b;
  43.  
  44.     // ((3 + 1) * 2) - 1 = 7
  45.     println!("results: {}", chain_2(0, &|context| {
  46.         chain_1(context, &|_| {
  47.             panic!("Error!")
  48.         })
  49.     }));
  50. }
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