I am using the "mlrMBO" library in R (CRAN - Package mlrMBO). I am trying to learn how to run an optimization algorithm on some arbitrary function that I defined:

```
#Load library
library(mlrMBO)
#define objective function
obj.fn = makeMultiObjectiveFunction(
name = "My test function",
fn = function(x1, x2, x3, x4) {
var_1 <- sin(x1 + x2)
var_2 <- cos(x1 - x2)
var_3 <- x1 + x4
var_4 <- x3 + x4 -7
goal_1 = sum(var_1 + var_2 + var_3 + var_4)
goal_2 = var_1 + var_2 - var_3 + var_4
goal_3 = var_1 + var_2 - var_3 + 2*var_4
return(c(goal_1, goal_2, goal_3))
},
#define acceptable ranges
par.set = makeParamSet(
makeNumericParam("x1", lower = 20, upper = 40),
makeNumericParam("x2", lower = 30, upper = 45),
makeNumericParam("x3", lower = 10, upper = 20),
makeNumericParam("x4", lower = 10, upper = 50),
#define constraints
forbidden = expression(x2 >x1 | x3 > x4)
),
minimize=TRUE
)
#create control gird
control=makeMBOControl(propose.points=1, final.method="best.predicted", final.evals=10)
control=setMBOControlTermination(control, iters=10)
control=setMBOControlInfill(control, crit=makeMBOInfillCritEI())
#perform optimization
lrn=makeMBOLearner(control, obj.fun)
```

This code returns the following error:

```
Error in fn(unlist(x), ...) : unused argument (unlist(x))
```

Does anyone know why this error is being produced? I have tried to look at various reference material (e.g. https://cran.r-project.org/web/packages/mlrMBO/mlrMBO.pdf ), but so far I am unable to understand why this error is being produced.

Can someone please show me what am I doing wrong?

Thanks