Type Checking

The type checker is to determine if the program is well-typed or not.

• It takes the AST generated from the parser.
• update the environment each time we define a new variable or function.
• generate a list of typing equations that should hold for our term to be well typed.

We do recursive traversal of the AST and generate the equivalent equation, and when we have a Var type the typing is done relative to the typing environment that map between identifiers and its type.
Then we check the correctness for each equation to determine if our program is well typed or not.
If it's not well typed we throw a failwith exception with the right error message.

Initially, the environment contains the types of the predefined function ("print_int", "print_float")

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