The Neper language in brief

This page condenses the essentials. The normative references are docs/grammar.ebnf (revision 3), docs/llm-neper-card.md (the editing card this page is derived from), docs/spec.md and docs/module-apis.md (the standard library index).

Program shape

use e.io

fn main(a: *mem.Arena, args: []str) -> err {
    try io.print("Hello, Neper")
    ret ok
}

A program is a run of declarations: use, const, var, error, type, fn and extern. Modules are named by dotted paths (e.io, e.algo.graph) and imported with use.

Bindings and types

let binds once, var mutates. A binding is typed explicitly (var n: u32 = 0u32) or inferred from its literal.
Literals carry their type: 1u64, 1.5f64, u8(200), true, nil, zero.
Casts are explicit and checked: i64(x), u32(n), f64(i); a narrowing cast traps out of range in debug builds.
Strings are byte slices (str) with escapes limited to \n \t \r \\ \" \' \0 \xHH; use the modules for Unicode work.

Errors, `try` and `defer`

error declares an error set; functions answer err and return ret ok or ret Name.
try f() propagates an error, try f() as a statement or let (v, e) = f() when the answer is a tuple.
defer runs at scope exit; resources such as files and windows are resource types closed by the runtime or an explicit close.

Structs, enums and unions

struct fields are named and typed; literals are Type { field: value }.
enum is an integer with named members; switch is exhaustive over them.
union is the C-layout escape hatch for extern ABIs; the safe tagged form is enum with payload arms.

Comptime and generics

fn largest[T: type](xs: []const T) -> T { ... }

Type parameters are written [T: type]; functions and types with compile-time arguments are monomorphised. Modules expose generic helpers (e.g. channel.send[i64]), and comptime parameters carry array lengths and flags.

No-shadowing, and reserved words

A local may not reuse a module-scope function, type or module name (the E-NAME-0003 diagnostic). The full keyword list is in the editing card: as break case const continue default defer else enum error extern false fn for if in isize let nil nocheck ok own resource ret shared struct switch true try type undef union unreachable use usize var when while zero.

Standard library

Everything is in modules under e.: e.io, e.mem, e.str, e.net.http, e.game, e.ui, e.chart, e.algo.*, e.math.* and more. The complete index is docs/module-apis.md; the machine-readable summary is llms.txt.

Install Neper, then read the get-started walkthrough and the measured blog posts.