Invalid type passed!
Received:
{type}
Expected one of:
{asideVariants.join(", ")}
: <>
{title &&
{title}
}
{children}
}
;
};
Tolk supports exceptions and handling them with `try catch`. If an exception is not caught, the program terminates with `errCode`.
In [TVM](/tvm/overview), exceptions are numbers. There is no `Error` class or exception hierarchy, only [exit codes](/tvm/exit-codes).
## Error codes
A simple pattern is to define a constant for each exception that a contract may produce:
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
const ERR_LOW_BALANCE = 200
const ERR_SIGNATURE_MISMATCH = 201
```
Yet, this pattern should not be used in production environments, as it does not scale. Instead, use an `enum` — they are easier to maintain and reference, as enumerations come with exhaustive checks from the compiler:
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
enum ErrCode {
LowBalance = 200,
SignatureMismatch, // implicitly 201
}
```
Use these constants in `throw` and related statements described on this page.
## `throw` statement
To throw an exception unconditionally:
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
throw ERR_CODE;
```
Non-constant expressions such as `throw someVariable` are supported but not recommended. It works, but the compiler cannot determine possible error codes and therefore cannot provide a correct ABI for external callers.
An exception can carry an argument of type [`unknown`](/languages/tolk/types/unknown):
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
throw (ERR_CODE, errArg);
```
The argument is available in `catch`. Any type can be passed; multi-slot types are packed into a single TVM slot automatically:
## `assert` statement
An `assert` is a shorthand for "throw if a condition is not satisfied". It is commonly used when parsing user input:
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
assert (msg.seqno == storage.seqno) throw E_INVALID_SEQNO;
assert (msg.validUntil > blockchain.now()) throw E_EXPIRED;
```
* The long form `assert (condition) throw ERR_CODE` is preferred.
* The short form `assert (condition, ERR_CODE)` exists but is not recommended.
The `condition` must be either a boolean or an integer, where non-zero integers are treated as `true` and all other integer values as `false`.
An `assert` statement is equivalent to:
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
if (!condition) {
throw ERR_CODE;
}
```
## TVM implicit throws
During contract execution, TVM may throw runtime exceptions. For example:
* `slice.loadInt(8)` will fail when the slice is empty.
* `builder.storeRef(cell)` will fail when the builder has 4 references already.
* `tuple.push(value)` will fail when the tuple has 255 elements already.
* etc.
Additionally, an [out-of-gas exception](/tvm/exit-codes#14%3A-out-of-gas-error) may occur at **any** point, and it cannot be caught using a [`catch`](#try-catch-statement) block. It can only be prevented by writing [exhaustive testing suites](/contract-dev/testing/overview).
## `try catch` statement
The `catch` block is used to handle most runtime errors within the `try` block, except for out-of-gas exceptions:
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
try {
// ...
} catch (errCode) {
// errCode is `int`
}
```
* Use the short form `catch { ... }` when `errCode` is not needed.
* Use the long form `catch (errCode, arg)` when the exception may carry an argument produced by `throw (errCode, arg)`. The `arg` is of type `unknown`; use `as` to cast it. If the exception was thrown as `throw errCode`, the argument is `null`.
```tolk theme={"theme":{"light":"github-light-default","dark":"dark-plus"},"languages":{"custom":["/resources/grammars/tolk.tmLanguage.json","/resources/grammars/tlb.tmLanguage.json","/resources/grammars/fift.tmLanguage.json","/resources/grammars/tasm.tmLanguage.json","/resources/grammars/func.tmLanguage.json"]}}
try {
throw (ERR_LOW_BALANCE, 555);
} catch (errCode, arg) {
val data = arg as int; // 555
}
// even complex types can be thrown
try {
throw (ERR_NOT_FOUND, Point { x: 10, y: 20 });
} catch (errCode, arg) {
val p = arg as Point; // { x: 10, y: 20 }
}
```