# E4151

Values of type `FuncRef[T]` must be closed function -- it should not capture any local variables.

MoonBit provides a builtin type `FuncRef[T]`, which represents closed function of type `T`.
In MoonBit's native backend, `FuncRef[T]` corresponds to function pointer type,
so `FuncRef[T]` can be passed to C FFI functions requiring a callback directly, for example setting UNIX signal handler.
Values of type `FuncRef[T]` must be closed function type,
if a function which captures free variables or a non-function is supplied, an error will be reported.

## Erroneous example
```{literalinclude} /sources/error_codes/4151_error/top.mbt
:language: moonbit
```

## Suggestion
If your callback function really needs to have captures,
consider adding the captures as extra parameters to the function.

Otherwise, use a closed function that does not capture local variables:

```{literalinclude} /sources/error_codes/4151_fixed/top.mbt
:language: moonbit
```