## The new type checker Snowcat

WARNING: Snowcat is our type checker starting with Apalache version 0.15.0. If you are using Apalache prior to version 0.15.0, check the chapter on old type annotations.

### How to write type annotations

Check the HOWTO. You can find detailed syntax of type annotations in ADR002.

### How to run the type checker

The type checker can be run as follows:

\$ apalache typecheck [--infer-poly=<bool>] <myspec>.tla


The arguments are as follows:

• General parameters:
• --infer-poly controls whether the type checker should infer polymorphic types. As many specs do not need polymorphism, you can set this option to false. The default value is true.

There is not much to explain about running the tool. When it successfully finds the types of all expressions, it reports:

 > Running Snowcat .::..
> Your types are great!
...
Type checker [OK]


When the type checker finds an error, it explains the error like that:

 > Running Snowcat .::.
[QueensTyped.tla:42:44-42:61]: Mismatch in argument types. Expected: (Seq(Int)) => Bool
[QueensTyped.tla:42:14-42:63]: Error when computing the type of Solutions
> Snowcat asks you to fix the types. Meow.
Type checker [FAILED]