@kha I added this example as a template for what the equation compiler
will have to do. The plan is:
- We can use `partial` to define any function if the result type is
inhabited.
- If the result type is of the form `Partial a`, the equation compiler
generates lemmas of the form:
```
theorem fooEq args : terminates (foo args) → foo args = lhs
```
The new test contains an example.
@kha: I initially planned to use the UTF8 API only in very special
cases, but I found them to be super useful. They allow us to implement
an efficient String library mostly in Lean.
However, the there was a problem: `abbrev String.Pos := USize`.
This definition is fine for a low level API, but this is not the case
anymore. By having `String.Pos := USize`, we will not be able to
prove natural theorems for the `String` API. For example,
`String.map id s = s` did not hold. We would have to include the
artificial antecedent `s.length <= usizeMax` (or something like this).
I suspect it would be very painful.
So, this commit defines `String.Pos` as `Nat`. The performance
overhead seems to be very small.
In 64-bit machines, the max small nat value should now be (2^63 - 1), and on 32-bit
machines (2^32 - 1).
The main motivation for this modification are the array indexing
operations. With the new representation, if a Nat index is not small,
then it must not be a valid index. This was not true in 64-bit
machines. Example: an array of size 2^33 would fit in memory, and but
an index `i` > 2^32 - 1 would not be a small nat value.
After we erase types and proofs, `Decidable.toBool` can be replaced with
the identity function since `Decidable A` and `Bool` have the same
runtime representation. By eagerly expanding `toBool`, we introduce
unnecessary `cases` expressions.
`elim_jp1_fn` was incorrectly expanding join points that were used more
than once. The issue is that the `foreach` combinator "may" skip nodes
that have already been visited.
The `offset` field is problematic because it prevents us from having an
efficient way of moving back and forth between `String.Pos` and `String.Iterator`.
@kha I temporarily added `String.OldIterator` for making sure the
parser doesn't break. This is a temporary fix that will be eliminated
after we replace `parsec`.
@kha It will be awesome to automate the following idiom with a macro :)
```
def defIndent := 4
def getIndent (o : Options) : Nat := o.get `format.indent defIndent
@[init] def indentOption : IO Unit :=
registerOption `format.indent { defValue := defIndent, group := "format", descr := "indentation" }
```
We can now write
```
@[init] def initFn : IO Unit := ...
```
instead of
```
def initFn : IO Unit := ...
@[init initFn] constant execInitFn : Unit := ()
```
We can use this primitive to process command line arguments of the form
`-D <key> = <value>`
TODO: allow users to attach `[init]` to definitions of the form
```
@[init] def foo : IO Unit := ...
```
and avoid the awkward auxiliary constant.
