Language reference
Every construct in slate, written down once, in its own place.
The pages
| Lexical structure | files, comments, indentation, names, literals, interpolation |
| Values | the eleven kinds, truth, equality, conversion |
| Expressions | operators, the precedence table, ranges, with, ??, ?., spread |
| Statements | val and var, assignment, if, every loop, blocks |
| Patterns | match, is, what binds, what tests, exhaustiveness |
| Functions | definitions, lambdas, defaults, named arguments, ...rest |
| Types | type, annotations, what the checker will and will not say |
| Opaque types | opaque type — a representation only its own file may treat as one |
| Objects | fields, proto, the receiver rule, operator methods |
| Collections | Set and Map, any value as a key, size, iteration |
| Classes | class, from, is, new, class patterns |
| Data types | data, variants, immutability, exhaustive match |
| Modules | export, import, the three kinds of specifier |
| Asynchrony | async, await, promises, the two queues, generators |
| Faults | throw, try/catch, and results as the other channel |
| Elements | slx — <div> in the source, and what it desugars to |
| Tests | @test, slate test, the assertions |
| Packages | the manifest, slate install, the cache, slate.sum |
| JavaScript | slate js, and where the two back ends differ |
| External | external, and what crosses the boundary to the host |
How to read a rule here
A rule about what slate refuses is a rule about what it refuses to COMPILE unless the page says otherwise. slate has two moments at which a program can be told it is wrong, and they are not interchangeable: the compile, which happens before anything runs and may only refuse a program that would genuinely fail; and the run, which is where a dynamically typed language does most of its checking. A page says which.
Two failure channels run through the whole language, and knowing which is which explains most of the library. A condition the caller was always going to deal with — a file that is not there, a connection refused, text that will not parse — is an answer: a result object the caller has to look at. A defect in the program — the wrong kind of argument, a division by zero, a value with no JSON form — is a fault, which unwinds until something catches it. Neither is used for the other’s job anywhere in slate.
slate refuses to store absence. There is no undefined; null is a value like any other and is
the only one. Nothing in the language can put “no value at all” into an array, an object field, a
parameter or a variable — which is why pop on an empty array faults rather than answering nothing,
why a parameter nobody gave is not bound rather than bound to a sentinel, and why an optional object
field is a shape question rather than a value.