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Hacking

Some notes on modifying Chawan’s code.

Style

Refer to the NEP1 for the basics. Also, try to keep the style of existing code.

In particular, style of vendored libraries should not be changed unless upstream is dead.

Casing

Everything is camelCase. Enums are camelCase too, but the first part is an abbreviation of the type name. e.g. members of SomeEnum start with se.

Exceptions:

Rationale: consistency.

Wrapping

80 chars per line.

Exceptions: URL comments.

Rationale: makes it easier to edit in vi.

Spacing

No blank lines inside procedures (and other code blocks). A single blank line separates two procs, type defs, etc. If your proc doesn’t fit on two 24-line screens, split it up into more procs instead of inserting blank lines.

Exceptions: occasionally a proc will get larger than two screens, and that’s ok, but try to avoid it.

Rationale: makes it easier to edit in vi.

Param separation

Semicolons, not commas. e.g.

# Good
proc foo(p1: int; p2, p3: string; p4 = true)
# Bad
proc bar(p1: int, p2, p3: string, p4 = true)

Rationale: makes it easier to edit in vi.

Naming

Prefer short names. Don’t copy verbose naming from the standard.

Rationale: we aren’t a fruit company.

Comments

Comment what is not obvious, don’t comment what is obvious. Whether something is obvious or not is left to your judgment.

Don’t paste standard prose into the code unless you’re making a point. If you do, abridge the prose.

Rationale: common sense, copyright.

C style

Note that most C code included in this repository is from vendored external libraries like QuickJS; following points only apply to code we own, such as chaseccomp.

JavaScript style

Similar to C style.

Note: although QJS has an optimizer, compared to that of a static language it is very limited. Some ugliness is acceptable if it results in more efficient code.

Features to avoid

List of Nim features/patterns that sound like a good idea but aren’t, for non-obvious reasons.

Exceptions

Avoid at all costs. If a standard library procedure throws, do not use it. Also avoid try...finally (even without catch).

For own procs, use Result/Opt instead. For large objects this may result in excessive copying; to avoid inefficient code, either make them ref or use manual RVO:

proc blah(input: int; output: var BigObject): Opt[void] =
  if input < 0:
    return err()
  output = BigObject(...)
  ok()

In new modules, always specify:

# This is an example module.
# After any header comments, but before imports:

{.push raises: [].}

import ...

# At the end of the file:
{.pop.} # raises: []

Implicit initialization

Avoid. The correct way to create an object:

let myObj = MyObject(
  param1: x,
  param2: y
  # if there's e.g. a param3 too, it's OK to leave it out and let it be
  # default initialized.
)

For primitive types, just set them to 0, ““, etc.

As a special case, implicit initialization is allowed for arrays:

var buf1: array[1234, char] # when you need 0-initialization
var buf2 {.noinit.}: array[1234, char] # when you don't need 0-initialization

Note that the second example is unsafe and doesn’t work with GC’ed types, so don’t use it unless you know what you’re doing.

out parameters

out parameters crash the 1.6.14 compiler. Use var instead.

Copying operations

substr and x[n..m] copies. Try to use toOpenArray instead, which is a non-copying slice. (Obviously, you should use substr if you need to copy.)

Note that = usually copies. If you’re copying a large object a lot, you may want to set its type to ref. For seq/string you can also try using move, but be sure to check the generated code because often times it doesn’t work in refc.

Beware of pairs on sequences of objects; it copies. Use mypairs if you don’t need mutation, mpairs if you do:

proc foo(objs: openArray[SomeObj]) =
  for i, obj in objs: # Bad. this copies. obj's type is "SomeObj".
    obj.bar(i)
  # import utils/twtstr to use mypairs.
  for i, obj in objs.mypairs: # OK, doesn't copy. obj's type is "lent "SomeObj".
    obj.bar(i)
  for i, obj in objs.mpairs: # OK, doesn't copy. obj's type is "var SomeObj".
    obj.i = i

func, .noSideEffect

These introduce function coloring for little to no benefit.

Just use proc without .noSideEffect.

Closures

A closure is an anonymous proc that uses (“captures”) variables in its wrapper proc. These introduce implicit GC’ed environments which are often wasteful, and combined with JS they are an easy way to produce memory leaks.

To enforce this rule, proc variables or members always have a nimcall (or cdecl) pragma. The environment can be passed explicitly, e.g.:

type
  FooCallback = proc(foo: Foo; opaque: RootRef) {.nimcall, raises: [].}

  Foo = ref object
    callback: FooCallback
    opaque: RootRef # add an explicit environment like this if needed

Fixing cyclic imports

In Nim, you can’t have circular dependencies between modules. This gets unwieldy as the HTML/DOM/etc. specs are a huge cyclic OOP mess.

The preferred workaround is global function pointer variables:

# Forward declaration hack
var forwardDeclImpl*: proc(window: Window; x, y: int) {.nimcall, raises: [].}
# in the other module:
forwardDeclImpl = proc(window: Window; x, y: int) =
  # [...]

Don’t forget to make it .nimcall, and to comment “Forward declaration hack” above. (Hopefully we can remove these once Nim supports cyclic module dependencies.)

Debugging

Note: following text assumes you are compiling in debug mode, i.e. make TARGET=debug.

The universal debugger

“eprint x, y” prints x, y to stderr, space separated.

Normally you can view what you printed through the M-c M-c (escape + c twice) console. Except when you’re printing from the pager, then do cha [...] 2>a and check the “a” file.

Sometimes, printing to the console triggers a self-feeding loop of printing to the console. To avoid this, disable the console buffer: cha [...] -o start.console-buffer=false 2>a. Then check the “a” file.

You can also inspect open buffers from the console. Note that you must run these before switching to the console buffer (i.e. before the second M-c), or it will show info about the console buffer.

gdb

gdb should work fine too. You can attach it to buffers by putting a long sleep call in runBuffer, then retrieving the PID as described above. Note that this will upset seccomp, so you should compile with make TARGET=debug DANGER_DISABLE_SANDBOX=1.

Debugging layout bugs

One possible workflow:

The -o start.console-buffer=false trick (see above) is especially useful when debugging a flow layout path that the console buffer also needs.

Don’t forget to add a test case after the fix:

$ ./test/layout/add test/layout/my-test-case.html

Sandbox violations

You got a syscall number (assuming you’re on Linux); look it up in the Linux syscall table for your architecture.

To get more context on what happened, you can run strace -f ./cha -o start.console-buffer=false [...] 2>a, trigger the crash, then search for the last occurrence of “— SIGSYS”. Then search backwards on the PID to see the last syscalls.

(strace also shows the syscall name, so it may be easier to check like that than looking it up in the syscall table.)

Resources

You may find these links useful.

Chawan

Of particular interest in the documentation are:

WHATWG

Note that some of these are updated daily, such as the HTML standard.

CSS standards

Good news is that unlike WHATWG specs, this doesn’t change daily. Bad news is that CSS 2.1 was the last real CSS version, and newer features are spread across a bunch of random documents with questionable status of stability: https://www.w3.org/Style/CSS/specs.en.html.

Other standards

It’s unlikely that you will need these, but for completeness’ sake:

Nim docs

MDN

https://developer.mozilla.org/en-US/docs/Web

MDN is useful if you don’t quite understand how a certain feature is supposed to work. It also has links to relevant standards in page footers.