ziglings/exercises/024_errors4.zig

//
// Using `catch` to replace an error with a default value is a bit
// of a blunt instrument since it doesn't matter what the error is.
//
// Catch lets us capture the error value and perform additional
// actions with this form:
//
//     canFail() catch |err| {
//         if (err == FishError.TunaMalfunction) {
//             ...
//         }
//     };
//
const std = @import("std");

const MyNumberError = error{
  TooSmall,
  TooBig,
};

pub fn main() void{
  // The "catch 0" below is a temporary hack to deal with
  // makeJustRight()'s returned error union (for now).
  const a: u32 = makeJustRight(44) catch 0;
  const b: u32 = makeJustRight(14) catch 0;
  const c: u32 = makeJustRight(4) catch 0;
  
  std.debug.print("a={}, b={}, c={}\n", .{ a, b, c });
}

// In this silly example we've split the responsibility of making
// a number just right into four (!) functions:
//
//     makeJustRight()   Calls fixTooBig(), cannot fix any errors.
//     fixTooBig()       Calls fixTooSmall(), fixes TooBig errors.
//     fixTooSmall()     Calls detectProblems(), fixes TooSmall errors.
//     detectProblems()  Returns the number or an error.
//
fn makeJustRight(n: u32) MyNumberError!u32{
  return fixTooBig(n) catch |err|{
    return err;
  };
}

fn fixTooBig(n: u32) MyNumberError!u32{
  return fixTooSmall(n) catch |err|{
    if(err == MyNumberError.TooBig){
      return 20;
    }
    
    return err;
  };
}

fn fixTooSmall(n: u32) MyNumberError!u32{
  // Oh dear, this is missing a lot! But don't worry, it's nearly
  // identical to fixTooBig() above.
  //
  // If we get a TooSmall error, we should return 10.
  // If we get any other error, we should return that error.
  // Otherwise, we return the u32 number.
  return detectProblems(n) catch |err|{
    if(err == MyNumberError.TooSmall){
      return 10;
    }
    
    return err;
  };
}

fn detectProblems(n: u32) MyNumberError!u32{
  if(n < 10){ return MyNumberError.TooSmall; }
  if(n > 20){ return MyNumberError.TooBig; }
  return n;
}