Apr 27, 2020 · Apr 27, 2020 · Apr 27, 2020
diff --git a/README.md b/README.md
 # datalog-cpp

 implementation of datalog (without negation, and semi-naive bottom-up evaluation) in C++

 work-in-progress

 not yet alpha!

 ## Documentation

 - [Quick git how to](./docs/git-how-to.md)
diff --git a/docs/git-how-to.md b/docs/git-how-to.md
 # Quick git how to

 Quick instructions on how to contribute to this repo from the command line.

 Please see link to other resources below.

 ## 1. Clone the repo on your local machine

 ```
 git clone git@github.com:Z80coder/datalog-cpp.git
 ```

 You should now have a `datalog-cpp` folder.

 ```
 cd datalog-cpp
 ```

 ## 2. Update the master branch


 ```
 git checkout master
 git pull
 ```

 ## 3. Create a new branch for your work

 You will develop your code in your own branch. First you need to create it.

 ```
 git checkout -b your_branch_name

 git push --set-upstream origin your_branch_name
 ```

 ## 4. Or switch to your existing branch for your work

 If you already done the step above, then ensure you are in your branch when you write code.

 ```
 git checkout your_branch_name
 ```

 ## 5. Do some coding

 Work as normal

 ## 6. Commit your changes

 Type
 ```
 git status
 ```
 to check which files you've added or changed. Note the files you want to commit at this time, then:
 ```
 git add newfile1 newfile2
 ```
 Then commit the changes
 ```
 git commit
 ```

 ## 7. Push your changes to GitHub

 All you've done so far is declare some changes. You haven't yet pushed them to the GitHub repo. Let's do that now.

 ```
 git push
 ```

 You can keep adding, committing and pushing until you're ready to open a pull-request (with the intent of merging the changes in your branch into the `master` on the repo).

 ## 8. Open a pull-request

 Go to https://github.com/Z80coder/datalog-cpp

 Click on the `Branch` button and look for `your_branch_name`, and then click on it.

 You should see a `New pull request` button. Click it and follow the user-interface prompts to create a pull-request.

 Once reviewed, your branch will be merged. You can contribute a new collection of changes by returning to `Create a new branch for your work` above.

 ## Other resources

 - Microsoft Visual Studio has git support built in, and there's a GitHub plugin for Visual Studio to enable direct cloning from GitHub repos.

 - Microsoft Visual Code also has plug-in extensions that support Git

 - https://guides.github.com/ has some useful guides.

 - Also, there's a GitHub Desktop app: https://desktop.github.com/
diff --git a/src/Variable.h b/src/Variable.h
 template <typename T>
 struct Variable : optional<T>
 {
    /**
     * @brief bind this variable to the supplied value (making it an unfree variable)
     *
     * @param value
     */
    void bind(const T &value)
    {
        this->emplace(value);
    }

    /**
     * @brief unbinds this variable (making it a free variable)
     *
     */
    void unbind()
    {
        this->reset();
    }

    /**
     * @brief checks whether this variable is free or bound
     *
     * @return true if bound to a value
     * @return false if free
     */
    bool isBound() const
    {
        return this->has_value();
    }

    const T &
    value() const
    /**
     * @brief returns the bound value (if not bound then throws an exception)
     *
     * @return const T&
     */
    const T &value() const
    {
        return this->optional<T>::value();
    }
 };

 // TODO: use auto more for return type of functions

 template <typename T>
 Variable<T> *var()
 {
Original file line number	Diff line number	Diff line change
		@@ -1,6 +1,11 @@
		# datalog-cpp

		implementation of datalog (without negation, and semi-naive bottom-up evaluation) in C++

		work-in-progress

		not yet alpha!

		## Documentation

		- [Quick git how to](./docs/git-how-to.md)
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,92 @@
		# Quick git how to

		Quick instructions on how to contribute to this repo from the command line.

		Please see link to other resources below.

		## 1. Clone the repo on your local machine

		```
		git clone git@github.com:Z80coder/datalog-cpp.git
		```

		You should now have a `datalog-cpp` folder.

		```
		cd datalog-cpp
		```

		## 2. Update the master branch


		```
		git checkout master
		git pull
		```

		## 3. Create a new branch for your work

		You will develop your code in your own branch. First you need to create it.

		```
		git checkout -b your_branch_name

		git push --set-upstream origin your_branch_name
		```

		## 4. Or switch to your existing branch for your work

		If you already done the step above, then ensure you are in your branch when you write code.

		```
		git checkout your_branch_name
		```

		## 5. Do some coding

		Work as normal

		## 6. Commit your changes

		Type
		```
		git status
		```
		to check which files you've added or changed. Note the files you want to commit at this time, then:
		```
		git add newfile1 newfile2
		```
		Then commit the changes
		```
		git commit
		```

		## 7. Push your changes to GitHub

		All you've done so far is declare some changes. You haven't yet pushed them to the GitHub repo. Let's do that now.

		```
		git push
		```

		You can keep adding, committing and pushing until you're ready to open a pull-request (with the intent of merging the changes in your branch into the `master` on the repo).

		## 8. Open a pull-request

		Go to https://github.com/Z80coder/datalog-cpp

		Click on the `Branch` button and look for `your_branch_name`, and then click on it.

		You should see a `New pull request` button. Click it and follow the user-interface prompts to create a pull-request.

		Once reviewed, your branch will be merged. You can contribute a new collection of changes by returning to `Create a new branch for your work` above.

		## Other resources

		- Microsoft Visual Studio has git support built in, and there's a GitHub plugin for Visual Studio to enable direct cloning from GitHub repos.

		- Microsoft Visual Code also has plug-in extensions that support Git

		- https://guides.github.com/ has some useful guides.

		- Also, there's a GitHub Desktop app: https://desktop.github.com/
Original file line number	Diff line number	Diff line change
Expand Up		@@ -15,30 +15,47 @@ using namespace std;
		template <typename T>
		struct Variable : optional<T>
		{
		/**
		* @brief bind this variable to the supplied value (making it an unfree variable)
		*
		* @param value
		*/
		void bind(const T &value)
		{
		this->emplace(value);
		}

		/**
		* @brief unbinds this variable (making it a free variable)
		*
		*/
		void unbind()
		{
		this->reset();
		}

		/**
		* @brief checks whether this variable is free or bound
		*
		* @return true if bound to a value
		* @return false if free
		*/
		bool isBound() const
		{
		return this->has_value();
		}

		const T &
		value() const
		/**
		* @brief returns the bound value (if not bound then throws an exception)
		*
		* @return const T&
		*/
		const T &value() const
		{
		return this->optional<T>::value();
		}
		};

		// TODO: use auto more for return type of functions

		template <typename T>
		Variable<T> *var()
		{
Expand Down