Error reference › Runtime diagnostics

What it means

Mozhi is dynamically typed with per-operator type rules. Arithmetic requires numbers (string concatenation with `+` is the exception), comparisons need comparable types, and index types are fixed per container.

Trigger condition

An operator is applied to a value type it cannot work with — e.g. subtraction on strings, `!` on a non-boolean, ordering `null` values, indexing an array with a string.

Minimal failing example

echo("total: " - 5)

Verified fix

echo("total: " + string(5))

Use `+` for string concatenation and `string(...)` conversions to move between types.

What the interpreter prints

error[RUNTIME001]: type mismatch
  --> app.mz:1:16
 1 | echo("total: " - 5)
   |                 ^
   |
reason: (the exact problem, with positions)
help:   (a suggested fix appears here)
docs:   https://crossberry-in.github.io/mozhi-doc/errors/RUNTIME001

Suggested fix

Print the operands with `string(...)` first to see their types, then convert operands explicitly before the operation.

Related syntax

Operators, type conversions

Related diagnostics

RUNTIME003RUNTIME004

Reference links

• All error codes
• CLI reference — output flags and JSON schema
• Free AI Diagnosis for this error

Limitations

The diagnostic names the operation in the message (as before v3.2.0) and adds the code/span; it does not yet print the operand types.