Add developer guide page

README.md

@@ -217,6 +217,9 @@ address an [open issue](https://github.com/Cambridge-ICCS/FTorch/issues), please
 [fork the repository](https://github.com/Cambridge-ICCS/FTorch/fork) and open a
 pull request.
 
+Detailed guidelines can be found in the
+[online developer documentation](page/developer.html).
+
 
 ### Code of Conduct
 Everyone participating in the _FTorch_ project, and in particular in the

pages/developer.md

@@ -0,0 +1,131 @@
+title: Developer Guide
+
+If you would like to contribute to the FTorch project, or modify the code at a deeper
+level, please see below for guidance.
+
+[TOC]
+
+
+## Getting involved
+
+Contributions and collaborations are welcome.
+
+For bugs, feature requests, and clear suggestions for improvement please
+[open an issue](https://github.com/Cambridge-ICCS/FTorch/issues/new/choose).
+
+If you have built something upon _FTorch_ that would be useful to others, or can
+address an [open issue](https://github.com/Cambridge-ICCS/FTorch/issues), please
+[fork the repository](https://github.com/Cambridge-ICCS/FTorch/fork) and open a
+pull request.
+
+
+### Code of Conduct
+Everyone participating in the FTorch project, and in particular in the
+issue tracker, pull requests, and social media activity, is expected to treat other
+people with respect and, more generally, to follow the guidelines articulated in the
+[Python Community Code of Conduct](https://www.python.org/psf/codeofconduct/).
+
+
+## Extending the API
+
+If you have a Torch functionality that you wish to bring in from the C++ API to
+the FTorch Fortran API the steps are generally as follows:
+
+* Modify `ctorch.cpp` to create a C++ version of the function that accesses `torch::<item>`.
+* Add the function to the header file `ctorch.h`
+* Modify `ftorch.fypp` to create a Fortran version of the function
+  that binds to the version in `ctorch.cpp`.
+
+Details of C++ functionalities available to be wrapped can be found
+in the [libtorch C++ API](https://pytorch.org/cppdocs/).
+
+As this is an open-source project we appreciate any contributions
+back from users that have extended the functionality.
+If you have done something but don't know where to start with
+open-source contributions please get in touch!<sup>*</sup>
+
+<sup>*</sup>_Our preferred method of contact is via Github issues and discussions,
+but if you are unfamiliar with this you can [email ICCS](mailto:jwa34@cam.ac.uk)
+asking for the FTorch developers._
+
+
+### Fortran source and Fypp
+
+The Fortran source code for FTorch is contained in `src/ftorch.f90`.
+However, this file should not be edited directly, but instead generated from
+`src/ftorch.fypp`.
+This is a file that is set up to be run through the
+[Fypp](https://fypp.readthedocs.io/en/stable/index.html) preprocessor.
+We use this because we want to create a pleasant interface of single function calls.
+The nature of Fortran means that this requires a lot of repeated combinations of
+array shapes and data types under interface structures.
+By using Fypp we can generate these programatically.
+
+Fypp can be installed via pip:
+```
+pip install fypp
+```
+
+To generate the Fortran code run:
+```
+fypp src/ftorch.fypp src/ftorch.f90
+```
+
+_Note: Generally it would be advisable to provide only the `.fypp` source code to
+reduce duplication and confusion. However, because it is a relatively small file
+and many of our users wish to _"clone-and-go"_ rather than develop, we provide both.<br>
+Development should only take place in `ftorch.fypp`, however._
+
+
+### git hook
+
+In order to streamline the process of uploading we provide a pre-commit hook in
+[`.githooks/pre-commit`](https://github.com/Cambridge-ICCS/FTorch/blob/main/.githooks/pre-commit).
+This will check that both the `.fypp` and `.f90` files have been updated together in a
+synchronous fashion before a commmit can take place.
+If this does not happen then the second line of defence (GitHub continuous integration)
+will fail following the commit.
+
+Use of the hook is not automatic and needs to be enabled by the developer
+(after they have inspected it and are happy with its contents).
+Hooks can be enabled by placing them in the `.git` directory with the following commands:
+```
+cp .githooks/pre-commit .git/hooks/
+chmod +x .git/pre-commit
+```
+
+### General guidelines
+
+* Match optional argument defaults between Fortran, C, and C++<br>
+  ([principle of least astonishment](https://en.wikipedia.org/wiki/Principle_of_least_astonishment)).
+* Handle `torch::Error` and `std::exception` in the C++ functions by catching and
+  printing to screen before exiting cleanly.
+
+
+## Documentation
+
+The API documentation for FTorch is generated using 
+[FORD](https://forddocs.readthedocs.io/en/latest/).
+For detailed information refer to the 
+[FORD User Guide](https://forddocs.readthedocs.io/en/latest/user_guide/index.html),
+but as a quick-start:
+
+* `!!` is used to signify documentation.
+* Documentation comes _after_ whatever it is documenting (inline or subsequent line).
+* Documentation can precede an item if designated using `!>`.
+
+FORD is pip installable:
+```
+pip install ford
+```
+To generate the documentation run:
+```
+ford FTorch.md
+```
+from the root of the repository.
+
+`FTorch.md` is the FORD index file, API documentation is automatically generated, and
+any further items are contained in `pages/` as markdown files.
+
+Documentation of the C functions in `ctorch.h` is provided
+by [Doxygen](https://www.doxygen.nl/index.html).

pages/index.md

@@ -7,5 +7,5 @@ _FTorch_.
 
 ## Useful resources
 
-* [The libtorch API](https://pytorch.org/cppdocs/api/library_root.html)
+* [The libtorch C++ API documentation](https://pytorch.org/cppdocs/)