documentation/docs/implementation/35-typechecking.md

Typechecking and code generation

TODO: Add notes here TODO: Talk about the queues that exist

Instructions

The process of code generation involves the production (creation) of instructions and consumption of them (consuming them and embedding them in other instructions). There are several types of instructions but the main important base ones are listed below.

The base Instruction

Every type of instruction that is produced during the code generation phase is a kind-of Instruction, it is the base class for all instructions and contains some common methods used by all of them:

1. setContext(Context)
   • Sets the Context object that is to be associated with this instruction.
   • This is normally done as a way to transfer the context from the respective parser-node to the corresponding instruction such that if such context is needed during further code generation (or even emit) it can then be accessed
2. Context getContext()
   • Returns this instruction’s associated context via its Context object
3. string produceToStrEnclose(string addInfo)
   • Returns a string containing the additional info provided through addInfo
   • The format of the returned string will be [Instruction: <className>: <addInfo>] where <className> is the name of the instruction type (kind-of) and <addInfo> as explained previously

Value-based instructions (Value)

TODO: Talk about the Value instruction base class here

A Value instruction is a kind-of Instruction of which represents code which generates some sort of value, think of literals, arithmetic operations, pointer dereferences, variable reads and so on. Every such instruction always has an associated Type object associated with it in order to know the intended type of the instruction. Below we show the API usage of the Value class:

1. Type getType()
   • Returns the type associated with this instruction
2. setType(Type)
   • Set the type to be associated with this instruction

There are many instructions which sub-type this Value class, these can be found in <TODO: Insert path here and put all Value-based instructions in their own module>.

---

Code generation

The method of code generation and type checking starts by being provided a so-called “action list” which is a linear array of dependency-nodes (or DNodes for code’s sake), this list is then iterated through by a for-loop, and each DNode is passed to a method called typeCheckThing(DNode):

foreach(DNode node; actionList)
{
    /* Type-check/code-gen this node */
    typeCheckThing(node);
}

The handling of every different instruction type and its associated typechecking requirements are handled in one huge if-statement within the typeCheckThing(DNode) method. This method will analyse a given dependency-node and perform the required typechecking by extracting the DNode’s emebedded parser-node, whilst doing so if a type check passes then code generation takes place by generating the corresponding instruction and adding this to some position in the code queue (discussed later).

Code queue

TODO: Add information on this

The code queue is used as a stack and a queue in order to facilitate instruction generation. Certain instructions are produced once off and then added to the back of the queue (“consuming” instructions) whilst other are produced and pushed onto the top of the queue (“producing” instructions) for consumption by other consuming instructions later.

An example of this would be the following T code which uses a binary operation with two operands (one being a LiteralValue instruction and the other being a FuncCall instruction):

                                1 + func()

This would result in a situation where we have the following production