# E4157

Unsupported signature in mbti file.

Currently, only abstract type, functions and methods are supported in mbti file.

## Erroneous example

```{literalinclude} /sources/error_codes/4157_error/E4157.mbti
:language: moonbit
```

## Suggestion

Use abstract type instead.

```{literalinclude} /sources/error_codes/4157_fixed/E4157-fixed.mbti
:language: moonbit
```