[fpc-devel] Return to pure functions!
J. Gareth Moreton
gareth at moreton-family.com
Thu Sep 19 20:25:48 CEST 2019
The Curious Kit is back! While I'm still waiting on my optimiser
overhaul and the node semantic pass to be approved (or rejected), I've
gone to taking a fresh look at pure functions, especially as I know more
about the node pass stage (after my work on the semantic pass).
I've managed to resolve the merge conflicts and the compiler at least
recognises the 'pure' directive and does some preliminary checks.
One thing I've fallen foul of is how to track variables in what will
become the 'node parser' that the compiler will use to evaluate pure
functions to calculate their return values. Because a variable can be
anything, I was planning on using the Variant type, mainly because it
contains a union that has everything I need in terms of recording what
type a variable is and how its value is stored, and I would only end up
duplicating that. However, the bootstrapper can't find the 'variants'
unit. Given that no other part of the compiler uses the Variant type,
is there a rule against using it, or a way to help Make find the
There's still a few things I'm working out, like originally I thought
that if a function has a 'var' parameter, then it cannot be pure, but
this is not always the case, because if the actual parameter (what
you're passing into it when calling said function) has a value that is
deterministic, then you can theoretically evaluate the function at
compile-time so long as it doesn't break any rules like accessing a
file. Most likely this will be an on-going project that I'll build in
segments, and I'll make a full design specification to go with it, at
least for my own benefit.
Submitting everything at once at the end may not be the best approach
for this, as I or others might find future optimisations for pure
functions later on (like ascending a pure function call outside of a
for-loop if it doesn't depend on the loop counter, so it is only called
once and its return value referenced as needed later on). My thought on
a first patch is to have the compiler at least recognise the 'pure'
directive even if it doesn't yet do anything. Is that permissible?
Gareth aka. Kit
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