I've been working on this slowly over several weeks, but it's too hard
to support 0 as the null value for addresses. I constantly have to add
exceptions for scalar value corresponding to an address type (now
occupying 2 locations). The final straw is the test for 'reload':
x:num <- reload text
'reload' returns an address. But there's no way to know that for
arbitrary instructions.
New plan: let's put this off for a bit and first create support for
literals. Then use 'null' instead of '0' for addresses everywhere. Then
it'll be easy to just change what 'null' means.
I have a plan for a way to avoid use-after-free errors without all the
overheads of maintaining refcounts. Has the nice side-effect of
requiring manual memory management. The Mu way is to leak memory by
default and build tools to help decide when and where to expend effort
plugging memory leaks. Arguably programs should be distributed with
summaries of their resource use characteristics.
Eliminating refcount maintenance reduces time to run tests by 30% for
`mu edit`:
this commit parent
mu test: 3.9s 4.5s
mu test edit: 2:38 3:48
Open questions:
- making reclamation easier; some sort of support for destructors
- reclaiming local scopes (which are allocated on the heap)
- should we support automatically reclaiming allocations inside them?
Useful for programming contests like https://halite.io
Doesn't suffer from C++'s usual buffered gotchas: it'll skip leading
whitespace. Slow, though. Can be speeded up, though.
- 20 minutes later
But what's the point? Typewriter mode is actually harder to test than
'raw' console mode. Writing Mu programs in typewriter mode is just going
to encourage us all to slack off on writing tests.
Always check if next_word() returned an empty string (if it hit eof).
Thanks Rebecca Allard for running into a crash when a .mu file ends with
'{' (without a following newline).
Open question: how to express the constraint that next_word() should
always check if its result is empty? Can *any* type system do that?!
Even the usual constraint that we must use a result isn't iron-clad: you
could save the result in a variable but then ignore it. Unless you go to
Go's extraordinary lengths of considering any dead code an error.
Better implementation of commit 3445: not requiring types for special
variables in scenarios. It turned out that it wasn't working anytime we
needed to call 'get' on a special variable inside a scenario. After
moving that work to an earlier transform we can now use 'filesystem'
without a type inside scenarios.
Undo 3272. The trouble with creating a new section for constants is that
there's no good place to order it since constants can be initialized
using globals as well as vice versa. And I don't want to add constraints
disallowing either side.
Instead, a new plan: always declare constants in the Globals section
using 'extern const' rather than just 'const', since otherwise constants
implicitly have internal linkage (http://stackoverflow.com/questions/14894698/why-does-extern-const-int-n-not-work-as-expected)
Move global constants into their own section since we seem to be having
trouble linking in 'extern const' variables when manually cleaving mu.cc
into separate compilation units.
Prefer preincrement operators wherever possible. Old versions of
compilers used to be better at optimizing them. Even if we don't care
about performance it's useful to make unary operators look like unary
operators wherever possible, and to distinguish the 'statement form'
which doesn't care about the value of the expression from the
postincrement which usually increments as a side-effect in some larger
computation (and so is worth avoiding except for some common idioms, or
perhaps even there).
Thanks Sam Putman for helping think through this idea.
When you encounter a backslash, strip it out and pass through any
following run of backslashes. If we 'escaped' a single following
character like C, then the character '\' would be the same as:
'\\' escaped once
'\\\\' escaped twice
'\\\\\\\\' escaped thrice (8 backslashes)
..and so on, the number of backslashes doubling each time. Instead, our
approach is to make the character '\' the same as:
'\\' escaped once
'\\\' escaped twice
'\\\\' escaped thrice
..and so on, the number of backslashes merely increasing by one each
time.
This approach only works as long as backslashes aren't also overloaded
to create special characters. So Mu doesn't follow C's approach of
overloading backslashes both to escape quote characters and also as a
notation for unprintable characters like '\n'.
Kartik Agaram