Command line

decl check schema.decl                       # parse + static checks, module-aware (exit 1 on errors)
decl evaluate site.decl                      # evaluate every output -> {"name": value, ...}
decl evaluate site.decl --output site        # one output -> its canonical JSON on stdout
decl evaluate site.decl --json               # {"ok", "value", "diagnostics"} report
decl evaluate cfg.decl --input deployed=doc.json --output deployed=out.json   # bind a document, write its completed value
decl validate cfg.decl --input deployed=doc.json     # bind a document to an input root (diagnostics only)
decl validate cfg.decl --input deployed=doc.json --expect-errors E4001
decl validate tests/validation               # judge a fixture corpus (valid/ and invalid/)
decl fmt src/*.decl                          # canonical formatting in place
decl fmt --check src/*.decl                  # exit 1 when a file is not canonical
decl-lsp                                     # stdio language server for editors

Diagnostics

Every problem is reported as a diagnostic:

cfg.decl: error [E4001] at deployed.port: out of range 1..65535
cfg.decl: error [E6001] TlsConfig.cert_present at deployed.tls: cert_path is required when tls is enabled
cfg.decl: warn [W6001] Service.scaled at deployed.workers: replicas 100 is outside the recommended range

file: severity [code] id at path: message. Codes are stable and registered in the specification (12. Errors); id names the assertion or the typed else message that fired; path is the canonical place in the document. With --json, the same fields arrive as a JSON array (check, validate) or inside the {ok, value, diagnostics} report (evaluate).

Failures never cascade: a member that fails to bind is invalidated, and everything that depends on it stays silent instead of reporting a second, misleading error (06. Constraints §6.7).

Editors

decl-lsp (from any of the three packages) speaks the Language Server Protocol over stdio and provides diagnostics, hover, and go-to-definition. Point your editor’s generic LSP client at it for .decl files:

• VS Code: a generic LSP client extension configured with command: decl-lsp, languageId: decl.
• Neovim: vim.lsp.start({ name = 'decl', cmd = { 'decl-lsp' }, root_dir = vim.fs.root(0, { 'decl.toml', '.git' }) }) from a FileType decl autocommand.
• Helix: add a [[language]] entry with language-servers = ["decl-lsp"] and [language-server.decl-lsp] command = "decl-lsp".

Syntax highlighting comes from the tree-sitter grammar in the repository (tree-sitter-decl/), which editors with tree-sitter support can load directly.