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Value: use std::span, change use of const

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**`Value` and `const`**

These two deserve some explanation. We'll get to lists later.

Values can normally be thought of as immutable, except they are
are also the vehicle for call by need, which must be implemented
using mutation.

This circumstance makes a `const Value` a rather useless thing:

 - If it's a thunk, you can't evaluate it, except by copying, but
   that would not be call by need.

 - If it's not a thunk, you know the type, so the method that
   acquired it for you should have returned something more specific,
   such as a `const Bindings &` (which actually does make sense
   because that's an immutable span of pointers to mutable `Value`s.

 - If you don't care about the type yet, you might establish the
   convention that `const Value` means `deepSeq`-ed data, but
   this is hardly useful and not actually as safe as you would
   supposedly want to trust it to be - just convention.

**Lists**

`std::span` is a tuple of pointer and size - just what we need.

We don't return them as `const Value`, because considering the
first bullet point we discussed before, we'd have to force all
the list values, which isn't what we want.

So what we end up with is a nice representation of a list in
weak head normal form: the spine is immutable, but the
items may need some evaluation later.
This commit is contained in:
Robert Hensing 2023-11-12 03:31:45 +01:00
parent 7055c65285
commit 260c614762
1 changed files with 8 additions and 29 deletions
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37
src/libexpr/value.hh
View File

@ -3,6 +3,7 @@
#include <cassert>
#include <climits>
#include <span>
#include "symbol-table.hh"
#include "value/context.hh"
@ -395,7 +396,13 @@ public:
return internalType == tList1 || internalType == tList2 ? smallList : bigList.elems;
}
const Value * const * listElems() const
std::span<Value * const> listItems() const
{
assert(isList());
return std::span<Value * const>(listElems(), listSize());
}
Value * const * listElems() const
{
return internalType == tList1 || internalType == tList2 ? smallList : bigList.elems;
}
@ -414,34 +421,6 @@ public:
*/
bool isTrivial() const;
auto listItems()
{
struct ListIterable
{
typedef Value * const * iterator;
iterator _begin, _end;
iterator begin() const { return _begin; }
iterator end() const { return _end; }
};
assert(isList());
auto begin = listElems();
return ListIterable { begin, begin + listSize() };
}
auto listItems() const
{
struct ConstListIterable
{
typedef const Value * const * iterator;
iterator _begin, _end;
iterator begin() const { return _begin; }
iterator end() const { return _end; }
};
assert(isList());
auto begin = listElems();
return ConstListIterable { begin, begin + listSize() };
}
SourcePath path() const
{
assert(internalType == tPath);