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This commit primarily changes how existential types are bound in
letbindings. Essentially, the constraints on both numeric and
existentially quantified types are lifted into the surrounding type
context automatically, so in
```
val f : nat -> nat
let x = f(3)
```
whereas x would have had type nat by default before, it'll now have
type atom('n) with a constraint that 'n >= 0 (where 'n is some fresh
type variable). This has several advantages: x can be passed to
functions expecting an atom argument, such as a vector indexing
operation without any clunky cast functions - ex_int, ex_nat, and
ex_range are no longer required. The let 'x = something() syntax is
also less needed, and is now only really required when we specifically
want a name to refer to x's type. This changes slightly the nature of
the type pattern syntax---whereas previously it was used to cause an
existential to be destructured, it now just provides names for an
automatically destructured binding. Usually however, this just works
the same.
Also:
- Fixed an issue where the rewrite_split_fun_constr_pats rewriting
pass didn't add type paramemters for newly added type variables in
generated function parameters.
- Updated string_of_ functions in ast_util to reflect syntax changes
- Fixed a C compilation issue where elements of union type
constructors were not being coerced between big integers and 64-bit
integers where appropriate
- Type annotations in patterns now generalise, rather than restrict
the type of the pattern. This should be safer and easier to handle
in the various backends. I don't think any code we had was relying
on this behaviour anyway.
- Add inequality operator to lib/flow.sail
- Fix an issue whereby top-level let bindings with annotations were
checked incorrectly
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