PEGO
Examples
Small grammars that show PEGO's features, with golden tests. Complete parsers for real languages, ready to import (CEL, CSV, CUE, DuckDB SQL, Go, JSON, Python, TypeScript, XML and YAML), are in parsers/; they are the larger examples.
| Directory | Contents | Features shown |
|---|---|---|
| calculator | A calculator for the four arithmetic operations and exponentiation. main.go evaluates the AST. |
Pratt expressions (calc.pego) compared with left recursion (calc_lr.pego); typed AST |
| outline | An outline whose hierarchy is expressed by indentation | Predicates and variables; captures in positive lookahead |
| minilang | A small programming language with functions, control flow and expressions | PEG statements nested with Pratt expressions; keyword exclusion; calls without a level |
Running
go run ./cmd/pego parse -g examples/minilang/minilang.pego -f sexpr < examples/minilang/testdata/fib.txt
go run ./examples/calculator "1 + 2 * (3 - 4) ^ 2"
Testing
examples_test.go parses every testdata/*.txt file in each directory with every .pego grammar in that directory and compares the result with the corresponding testdata/*.golden file.
Grammars in the same directory (such as calc.pego and calc_lr.pego) must produce the same results. Where a result legitimately depends on how a grammar is written, such as the expectations in a syntax error, a per-grammar golden file <input>.<grammar>.golden (for example error.calc_lr.golden) overrides the shared one.
go test ./examples/...
go test ./examples -update # regenerate the golden files (per-grammar files only where results differ)