# E4215

Compiler diagnostic name: `expect_template_string`.

Template writing expects a string template.

The `<+` template-writing operator writes a string template to the value on its
left. The right-hand side must be a string literal, an interpolated string, or a
multiline string template. Other expressions cannot be expanded as template
content.

## Erroneous example

```{literalinclude} /sources/error_codes/4215_error/top.mbt
:language: moonbit
```

The right-hand side is a variable reference, not a template string.

## Suggestion

Use a string template on the right-hand side.

```{literalinclude} /sources/error_codes/4215_fixed/top.mbt
:language: moonbit
```