# E4101

Unsupported expression after the pipe operator.

This error occurs when the expression after the pipe operator (`|>`) is not in a
supported form. The pipe operator allows you to chain function calls in a more
readable way, but only supports specific forms on its right-hand side.

The following forms are allowed after the pipe operator:

1. A single identifier (function name)
2. A regular function application (but not method calls)
3. A constructor name
4. A constructor application

## Erroneous example

```{literalinclude} /sources/error_codes/4101_error/top.mbt
:language: moonbit
```

## Suggestion

To fix this error, you can change the invalid pipe expression to normal function
or method application.

```{literalinclude} /sources/error_codes/4101_fixed/top.mbt
:language: moonbit
```