SAOC 2024 "Implementing Design Principles for Refactoring" Weekly update #2

[Dennis]

## Tasks Accomplished

### Implementing Design Principles for DMD Rafactoring
The goal was to apply design principles in the context of a 
compiler by separating the logic for semantic analysis from the 
structure of the Abstract Syntax Tree (AST) itself.

There are various types of design patterns but I focused on the 
**visitor's pattern** which is more robust and scalable
I started off by understanding the concept of Design pattern in 
software development, basically I would say design patterns are 
reusable solutions to common problems in software design.

**Facts about Design patterns**
- Design patterns can’t just be copied and pasted into a program, 
the way you can with off-the-shelf functions or libraries.
- Design pattern is not a specific piece of code, but a general 
concept for solving a particular problem. It's more like a 
process to solve a problem that suits the program or piece of 
code written.
- Design patterns are not **Algorithms**, although they are 
similar but totally different.
pattern provides a higher-level description of a solution, a 
particular design pattern can be applied differently to two 
coding problems. while an algorithm always defines a clear set of 
actions that can achieve some goal.

I also learnt that to use a design pattern effectively I should 
consider the following components in mind
1. Intent of the Pattern, describes what the pattern is meant to 
solve and how to solve it.
Just like the visitor pattern being used in the dmd codebase, the 
aim for it is to create new operations without changing the 
classes of the objects on which it operates.
2. Motivation, describes how useful the pattern is and what it 
tends to solve.
3. Structure of Classes, shows the correlation between the key 
classes and components involved in the pattern.
4. Code example, describes the implementation of the pattern in 
any language in this case D language.

### Using visitor's pattern
The visitor's pattern is one among several types of behavioral 
design pattern.
I can say that the Visitor pattern is a behavioral design pattern 
that lets you separate algorithms from the objects on which they 
operate.
**What does this mean?**
The Visitor pattern suggests that you create and place the new 
behavior into a separate class called **visitor** without adding 
it into existing classes. i.e the original object that had to 
perform the behavior is now passed to one of the visitor’s 
methods as an argument, providing the method access to all 
necessary data contained within the object.
This is exactly what I did in the [dsymbolsem.d 
file](https://github.com/dlang/dmd/pull/16880)


### Learning and implementing Uniform Function Call Syntax(UFCS)
I learnt about how useful this feature is, it basically allows 
functions to be called using method-call syntax on the object, 
even when the function isn't a member of the object's type.
**E.g** A normal function call `int result = foo(5);`
a UFCS call `int result = 5.foo();`
UFCS allows for a more fluent and extensible function call.


### Getting acquainted with the D language syntax
Still learning the D lang and improving my skills on it with the 
help of this [book](http://ddili.org/ders/d.en/index.html)


### Initial Refactoring of DMD AST
I've successfully decoupled the semantic functions from the AST 
file (attrib.d) to `dsymbolsem.d`
**Build was successful.** Pull request submitted 
[https://github.com/dlang/dmd/pull/16880](https://github.com/dlang/dmd/pull/16880)


### Challenges encountered earlier
- Although having little knowledge of how extensible UFCS is I 
got trapped in a simple error which I didn't realize
```
src/dmd/dsymbolsem.d(7557): Error: no property `newScope` for 
`super` of type `dmd.visitor.Visitor`
src/dmd/dsymbolsem.d(7557):    	the following error occured while 
looking for a UFCS match
src/dmd/dsymbolsem.d(7557): Error: function `newScope` is not 
callable using argument types `(Visitor, Scope*)`
src/dmd/dsymbolsem.d(7557):    	cannot pass argument `super` of 
type `dmd.visitor.Visitor` to parameter `Dsymbol d`
```
With the help of my mentor and a reviewer( Wilsonator) I 
understood that since it's a visitor,  newScope method does not 
exist there for visitors. So UFCS was used for this  
`dpd.newScope(sc)` instead of this
```
override void visit(DeprecatedDeclaration dpd)
	{
     	**auto scx = super.newScope(sc);**
```

- Encountered other errors which I fixed.

### Summary
This week laid a strong foundation for the separation of semantic 
routines from AST structure using the Visitor Pattern.

### Resources
- 
[https://medium.com/@digicore/software-design-patterns-101-a-beginners-guide-c6860ef8bb63](https://medium.com/@digicore/software-design-patterns-101-a-beginners-guide-c6860ef8bb63)
- 
[https://refactoring.guru/design-patterns/visitor](https://refactoring.guru/design-patterns/visitor)