Implement checks on Programs

[aochagavia]

We need to check the following properties (see Language.Fragment.hs at the bottom):

• All gotos target existing labels. Possible implementation: collect all goto targets into a set, collect all labels into a set, take the difference between gotos and labels (it should be empty)
• There is no dead code (eventually, we can pass a flag to allow it). It would be great to mention which labels are dead. Possible implementation: collect all gotos into a set, collect all labels into a set, take the difference between labels and gotos (it should be empty).
• There are no unused labels (ie labels without a corresponding fragment). Possible implementation: make an ordered list from the labels attached to each fragment, check if there are gaps between them (e.g. in [0, 1, 2, 5] we can see that labels 3 and 4 have been declared but don't have a corresponding fragment)

[ReinierMaas]

Create sets.

• [x] Collect all gotos into a set
• [x] Collect all labels into a set

All gotos target existing labels.

• [ ] Gotos should be a subset of labels, ERROR otherwise.

All labels should be used, no dead code.

• [ ] Labels should be a subset of gotos, WARNING otherwise.

What is the problem with skipped labels? If we already check for the above

[aochagavia]

"Collect all labels into a set" finds the labels present in the program. The point is that unused labels aren't present in the program.

[JorritDorrestijn]

The no unused labels check is implemented in commit f61be97949e9be24e0c40a6e008cb84ac714f076.

[ReinierMaas]

Unused labels already didn't end up in the source code.

[aochagavia]

@ReinierMaas the checks are performed on the Program data type, before translating it to code.

[ReinierMaas]

Okay, that clears that up. Checked the code this time, with tests!

[JorritDorrestijn]

The no dead check code is implemented in commit c788dc85418c0c9ba7333da1c6593c3da33b2ef5. This led to some code duplication which I've fixed as well.

[JorritDorrestijn]

It looks like all properties are checked now, so I'm closing this issue.