ZE-022 - Static Checks
Static Checks
This proposal has been accepted and implemented. You can use this feature since Zirric v0.1.0.
Introduction
Analysis reports mistakes it is certain about, instead of leaving them to fail at runtime.
Motivation
A typo, a miscounted argument or an operator with no meaning for its operands already stops a program — just later, without a position, and one at a time. Type hints were worse off still: the language asked for them and then checked nothing.
Proposed Solution
One rule governs everything: report only certainties. Anything the checker cannot see through fits, and only a combination it is sure about is rejected. In a language where types are optional, a false alarm costs more than a missed one.
Two kinds of thing are reported.
Certain runtime failures — the VM would stop:
| Reported | Example |
|---|---|
| undefined name | nosuchthing() |
| unknown module member | strings.lenght(…) |
| unknown field | person.nmae |
| wrong number of arguments | two(1) for fn two(a, b) |
| calling a non-callable | const x = 5 then x() |
| unsupported operator | P(1) + 3, -"a", !5 |
| indexing a non-indexable | 5[0] |
iterating without @Iterable |
for x <- 5 |
Broken declarations — the VM tolerates these, the program does not mean them:
| Reported | Example |
|---|---|
| argument type | fn(x: Int) called with a String |
| return type | fn f() -> Int { "s" } |
| declared value | const x: Int = "hello" |
| assignment | x = "hello" where x: Int |
| attribute constraint | an Int where @Iterable is declared |
Detailed Design
What is known
A check only fires when the checker knows a type. It knows:
| From | Type |
|---|---|
| a literal | its own |
| a type hint | what it names, following imports |
const |
its hint, else its value |
var |
its hint, else its value and every assignment — when they all agree |
if / switch expressions |
what every branch yields — when they agree, and a switch has a default |
| a collection literal | its elements — when they agree |
xs[0], d[k] |
the element or value type; a String or Binary gives a Byte |
| a call | the callee’s declared return type |
| a field, a module member | what it was declared as |
Everything else is unknown, and unknown fits everywhere.
Narrowing
Inside case is T: the value switched on is a T, so a branch that can only run for one member of a union is checked as that member.
Unions and attributes
A union fits when any member does, in either direction. An attribute constraint is satisfied when the type’s declaration carries that attribute.
Deliberately not reported
- A non-
Boolifcondition — the VM accepts it. - Element types in a mixed collection, which describe nothing.
- Anything depending on the order statements run in.
Changes to the Standard Library
None. Every module already passes.
Compatibility
Catching a runtime failure at compile time is not a breaking change. From here on, a new check needs a release note, not a proposal — provided it reports only certainties.
This amends ZE-017, which describes type hints as documentation that is never verified, including the element and key types of composite hints. Hints are still optional; written down, they are now binding.
