add proposed grammar doc
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doc/fwl_grammar.md
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doc/fwl_grammar.md
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# FWL Grammar Specification (MVP)
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## Overview
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FWL is a typed, functional DSL that compiles to nftables JSON. Programs are
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sequences of top-level declarations. The grammar uses explicit braces and
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semicolons throughout — no indentation sensitivity. Types are mandatory on all
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top-level declarations for MVP; inference is deferred to a later version.
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The target nftables table is a single table named `fwl` by default
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(configurable via a top-level `config` declaration). Both filter and NAT
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policies compile into this one table.
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---
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## Notation
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```
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::= production
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| alternative
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{ x } zero or more repetitions of x
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[ x ] optional x
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```
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String terminals are written in `"double quotes"`. Regex-like character classes
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use `[a-z]`, etc.
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---
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## Top-Level Structure
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```ebnf
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program ::= { config } { decl }
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config ::= "config" "{" { configProp ";" } "}"
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configProp ::= "table" "=" stringLit
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```
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Every non-`config` declaration is terminated by `";"`.
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---
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## Declarations
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```ebnf
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decl ::= interfaceDecl
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| zoneDecl
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| importDecl
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| letDecl
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| patternDecl
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| flowDecl
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| ruleDecl
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| policyDecl
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```
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### Interface
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```ebnf
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interfaceDecl ::= "interface" ident ":" ifaceKind "{" { ifaceProp ";" } "}" ";"
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ifaceKind ::= "WAN" | "LAN" | "WireGuard" | ident
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ifaceProp ::= "dynamic"
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| "cidr4" "=" cidrSet
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| "cidr6" "=" cidrSet
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cidrSet ::= "{" cidrLit { "," cidrLit } "}"
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```
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### Zone
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```ebnf
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zoneDecl ::= "zone" ident "=" "{" ident { "," ident } "}" ";"
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```
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### Import
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```ebnf
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importDecl ::= "import" ident ":" type "from" stringLit ";"
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```
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### Let
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```ebnf
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letDecl ::= "let" ident ":" type "=" expr ";"
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```
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### Pattern
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Named patterns are first-class; they may appear anywhere a structural pattern
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appears, including nested inside constructor patterns, `Frame(...)`, and other
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named patterns.
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```ebnf
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patternDecl ::= "pattern" ident ":" type "=" packetPat ";"
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```
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Named patterns are resolved during type-checking, not by macro-expanding at
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parse time. Recursive named patterns are a type error.
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### Flow
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```ebnf
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flowDecl ::= "flow" ident ":" "FlowPattern" "=" flowExpr ";"
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flowExpr ::= seqExpr
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seqExpr ::= flowAtom
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| flowAtom "." seqExpr
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| flowAtom "." seqExpr "within" duration
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```
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A `within` clause applies to the entire sequence to its left and binds most
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tightly to the innermost `.`. For MVP, `within` is only valid at the top level
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of a `flowExpr`.
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```ebnf
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flowAtom ::= ident
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```
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### Rule
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Rules are reusable, named packet-processing functions. They receive a `Frame`
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and return an `Action` (possibly via effects).
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```ebnf
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ruleDecl ::= "rule" ident ":" type "=" lambdaExpr ";"
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lambdaExpr ::= "\" ident "->" expr
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```
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A `rule` body must be a lambda at the top level for MVP.
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### Policy
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Policies are the entry points tied to nftables hooks. A policy body is a
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bare arm-block (no `case ... of` wrapper; the matched value is always the
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bound `Frame`-like parameter of the policy).
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```ebnf
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policyDecl ::= "policy" ident ":" type
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"on" "{" hookSpec "}"
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"=" armBlock ";"
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hookSpec ::= hookProp "," hookProp "," hookProp
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| hookProp "," hookProp "," hookProp "," -- trailing comma OK
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hookProp ::= "hook" "=" hook
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| "table" "=" tableName
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| "priority" "=" priority
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hook ::= "Input" | "Forward" | "Output" | "Prerouting" | "Postrouting"
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tableName ::= "Filter" | "NAT"
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priority ::= "Filter" | "DstNat" | "SrcNat" | "Mangle" | intLit
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```
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---
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## Arm Blocks
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Used uniformly inside `rule` bodies (via `case`) and `policy` bodies.
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```ebnf
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armBlock ::= "{" { arm } "}"
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arm ::= "|" pat guardOpt "->" expr ";"
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guardOpt ::= ε
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| "if" expr
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```
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---
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## Patterns
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Patterns describe packet structure and bind names. All membership/comparison
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predicates are guards (see § Expressions), not patterns — except for field
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constraints inside record patterns, which are written as field predicates.
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```ebnf
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pat ::= "_" -- wildcard
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| ident -- variable binding
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| namedPat -- first-class named pattern
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| ctorPat -- e.g., IPv4(ip, ...)
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| recordPat -- e.g., tcp { dport = :22 }
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| tuplePat -- e.g., (udp, payload)
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| framePat -- Frame(path, inner)
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| bytePat -- e.g., [0x01 _*]
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-- A named pattern reference; resolved at type-check time.
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-- Binds NO additional names itself (names are bound in the pattern's definition).
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namedPat ::= ident -- must refer to a declared pattern
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ctorPat ::= ident "(" patList ")"
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patList ::= pat { "," pat }
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recordPat ::= ident "{" [ fieldPatList ] "}"
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fieldPatList::= fieldPat { "," fieldPat }
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fieldPat ::= ident "=" literal -- equality constraint
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| ident -- bind field to same-named variable
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| ident "as" ident -- bind field to fresh variable
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tuplePat ::= "(" pat { "," pat } ")"
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-- Frame pattern: optional interface-path specifier, then inner packet pattern.
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framePat ::= "Frame" "(" [ pathPat "," ] pat ")"
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-- Interface-path: source, destination, or both.
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pathPat ::= endpointPat
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| endpointPat "->" endpointPat
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endpointPat ::= "_"
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| ident -- exact interface name
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| ident "in" ident -- interface is member of zone
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```
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**Note on `∈`:** the parser accepts both the Unicode `∈` and the ASCII keyword
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`in` as synonyms in all positions. The AST stores a single `MemberOf`
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constructor.
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### Byte Patterns
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Used in `pattern` declarations for payload matching.
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```ebnf
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bytePat ::= "[" { byteElem } "]"
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byteElem ::= hexByte -- e.g., 0x01
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| "_" -- any single byte
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| "_*" -- zero or more bytes
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```
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---
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## Expressions
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```ebnf
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expr ::= letExpr
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| ifExpr
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| doExpr
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| caseExpr
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| infixExpr
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letExpr ::= "let" ident "=" expr "in" expr
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ifExpr ::= "if" expr "then" expr "else" expr
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doExpr ::= "do" "{" doStmt { ";" doStmt } "}"
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doStmt ::= ident "<-" expr -- effectful bind
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| expr -- effectful sequence
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caseExpr ::= "case" expr "of" armBlock
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infixExpr ::= prefixExpr { infixOp prefixExpr }
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prefixExpr ::= appExpr
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| "!" prefixExpr
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appExpr ::= atom { atom } -- function application
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atom ::= ident
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| qualName
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| literal
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| tupleLit
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| setLit
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| mapLit
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| performExpr
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| "(" expr ")"
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performExpr ::= "perform" qualName "(" [ argList ] ")"
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tupleLit ::= "(" expr "," expr { "," expr } ")"
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setLit ::= "{" [ expr { "," expr } ] "}"
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mapLit ::= "{" mapEntry { "," mapEntry } "}"
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mapEntry ::= expr "->" expr
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argList ::= expr { "," expr }
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qualName ::= ident { "." ident }
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infixOp ::= "&&" | "||"
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| "==" | "!=" | "<" | "<=" | ">" | ">="
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| "in" | "∈" -- set/zone membership
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| "++" -- string/list concat
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| ">>" -- effect sequencing
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| ">>=" -- monadic bind
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```
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**Operator precedence** (high to low):
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| Level | Operators | Assoc |
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|-------|-----------------------|-------|
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| 7 | application | left |
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| 6 | `==` `!=` `<` `<=` `>` `>=` `in` `∈` | none |
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| 5 | `&&` | right |
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| 4 | `\|\|` | right |
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| 3 | `++` | right |
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| 2 | `>>=` | left |
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| 1 | `>>` | left |
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---
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## Types
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```ebnf
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type ::= funType
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funType ::= effectType
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| effectType "->" funType
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effectType ::= "<" [ ident { "," ident } ] ">" simpleType
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| simpleType
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simpleType ::= ident [ "<" typeList ">" ] -- parameterised type
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| "(" type { "," type } ")" -- tuple type
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| "(" type ")" -- grouped
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typeList ::= type { "," type }
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```
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Effect rows use angle brackets: `<FlowMatch, Log> Action`.
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For MVP, effect annotations are required on `rule` declarations that contain
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`perform` expressions and are optional on `policy` declarations.
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---
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## Actions
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`Action` is a built-in type. Its constructors are:
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```ebnf
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action ::= "Allow"
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| "Drop"
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| "Continue"
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| "Masquerade"
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| "DNAT" "(" expr ")"
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| "DNATMap" "(" expr ")"
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| "Log" "(" logLevel "," expr ")"
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logLevel ::= "Info" | "Warn" | "Error"
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```
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`Continue` is a legal action value and compiles to nothing (a no-op pass-
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through). It is used to make exhaustive arm blocks typecheck when earlier arms
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handle all interesting cases. A policy arm that returns `Continue` as the last
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arm is a type error (unreachable or missing terminator); a `rule` arm may
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return `Continue` to signal "pass control back to the caller."
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---
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## Effects
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The built-in effects available for MVP are:
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| Effect | Operations |
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|------------|---------------------------------------------------|
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| `FlowMatch`| `FlowMatch.check(flowId, pattern) : MatchResult` |
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| `Log` | `Log.emit(level, msg) : ()` |
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| `FIB` | `FIB.daddrLocal(ip) : Bool` |
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`MatchResult` constructors: `Matched`, `Unmatched`.
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Additional effects may be declared by the user in a future version.
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---
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## Literals
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```ebnf
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literal ::= intLit
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| stringLit
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| boolLit
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| ipv4Lit
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| ipv6Lit
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| cidrLit
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| portLit
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| durationLit
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| hexByte
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intLit ::= ["-"] digit+
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stringLit ::= '"' { strChar } '"'
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boolLit ::= "true" | "false"
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ipv4Lit ::= octet "." octet "." octet "." octet
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ipv6Lit ::= -- standard IPv6 notation including "::" compression
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cidrLit ::= (ipv4Lit | ipv6Lit) "/" digit+
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portLit ::= ":" digit+ -- e.g., :22, :8080
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durationLit ::= digit+ timeUnit
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timeUnit ::= "s" | "ms" | "m" | "h"
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hexByte ::= "0x" hexDigit hexDigit
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octet ::= digit+ -- 0-255
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```
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---
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## Lexical Rules
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```ebnf
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ident ::= letter { letter | digit | "_" }
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-- must not be a reserved word
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reserved ::= "config" | "interface" | "zone" | "import" | "let" | "in"
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| "pattern" | "flow" | "rule" | "policy" | "on"
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| "case" | "of" | "if" | "then" | "else" | "do"
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| "perform" | "within" | "as"
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| "WAN" | "LAN" | "WireGuard"
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| "Input" | "Forward" | "Output" | "Prerouting" | "Postrouting"
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| "Filter" | "NAT" | "Mangle" | "DstNat" | "SrcNat"
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| "Allow" | "Drop" | "Continue" | "Masquerade" | "DNAT"
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| "DNATMap" | "Log" | "Info" | "Warn" | "Error"
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| "Matched" | "Unmatched"
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| "dynamic" | "cidr4" | "cidr6" | "table" | "hook" | "priority"
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| "true" | "false"
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| "FlowPattern" | "Frame"
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comment ::= "--" { any char except newline }
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| "{-" { any char } "-}"
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whitespace ::= space | tab | newline | comment
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```
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Identifiers beginning with an uppercase letter are treated as constructor
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names by convention; those beginning with lowercase are variables. The lexer
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does not enforce this — it is a naming convention only, checked during
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type-checking.
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---
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## Resolved Inconsistencies from Proposal
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The following decisions were made to normalize the proposal's syntax:
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| Topic | Proposal state | MVP decision |
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|-------|---------------|--------------|
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| Interface body | Multiline, no delimiters | Braced block with `;` separators |
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| Policy body | `where` with indented arms | `=` followed by arm-block |
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| Rule body | `\frame -> case frame of \| ...` | `\ident -> expr`; `case` is a normal expression |
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| Policy vs rule | Distinct surface syntax | Policies use a bare arm-block; rules use `case` explicitly |
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| `Frame<{}>` | Unclear `<{}>` parameter | Parsed but ignored for MVP; written as `Frame` in practice |
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| Named patterns in sub-positions | Unclear | First-class everywhere; resolved at type-check time |
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| `∈` operator | Unicode only | Both `∈` and `in` accepted everywhere |
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| `Continue` | Unclear semantics | Legal `Action` constructor; compiles to nothing; type error if last arm of a policy block |
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| Single nftables table | Not specified | Default table name `fwl`; configurable via `config { table = "name"; }` |
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| `handle` syntax | Mentioned but unspecified | Deferred; MVP only has `perform` |
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| Effect annotations | Inconsistent (`<>` vs `{}`) | Angle brackets `<Eff1, Eff2>` everywhere |
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| Guard vs pattern membership | Mixed | Structural matching in patterns only; `in`/`∈` in guards only (except `fieldPat`) |
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---
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## Canonical Examples
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The following examples must all parse under the grammar above.
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### 1. Interface and Zone
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```fwl
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interface wan : WAN { dynamic; };
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interface lan : LAN {
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cidr4 = { 10.17.1.0/24 };
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cidr6 = { fe80::/10, fd12:3456::/48 };
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};
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interface wg0 : WireGuard {};
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zone lan_zone = { lan, wg0 };
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```
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### 2. Import and Let
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```fwl
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import rfc1918 : CIDRSet from "builtin:rfc1918";
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let forwards : Map<(Protocol, Port), (IP, Port)> = {
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(tcp, :8080) -> (10.17.1.10, :80),
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(tcp, :2222) -> (10.17.1.11, :22)
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};
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```
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### 3. Pattern and Flow
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||||
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||||
```fwl
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||||
pattern WGInitiation : (UDPHeader, Bytes) =
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(udp { length = 156 }, [0x01 _*]);
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pattern WGResponse : (UDPHeader, Bytes) =
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(udp { length = 100 }, [0x02 _*]);
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||||
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flow WireGuardHandshake : FlowPattern =
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WGInitiation . WGResponse within 5s;
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```
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||||
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### 4. Rule with Effects
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||||
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```fwl
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rule blockOutboundWG : Frame -> <FlowMatch, Log> Action =
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\frame ->
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case frame of {
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||||
| Frame(iif in lan_zone -> wan, IPv4(ip, UDP(udp, payload)))
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||||
if matches(WGInitiation, (udp, payload)) ->
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case perform FlowMatch.check(flowOf(ip, wg), WireGuardHandshake) of {
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||||
| Matched ->
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||||
do {
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||||
perform Log.emit(Warn, "WG blocked: " ++ show(ip.src));
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||||
Drop
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||||
};
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||||
| _ -> Continue;
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||||
};
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||||
| _ -> Continue;
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||||
};
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||||
```
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||||
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||||
### 5. Filter Policy
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||||
|
||||
```fwl
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||||
policy input : Frame
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||||
on { hook = Input, table = Filter, priority = Filter }
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||||
= {
|
||||
| _ if ct.state in { Established, Related } -> Allow;
|
||||
| Frame(lo, _) -> Allow;
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||||
| Frame(_, IPv6(ip6, ICMPv6(_, _)))
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||||
if ip6.src in fe80::/10 -> Allow;
|
||||
| Frame(_, IPv4(_, TCP(tcp, _)))
|
||||
if tcp.dport == :22 -> Allow;
|
||||
| Frame(_, IPv4(_, UDP(udp, _)))
|
||||
if udp.dport == :51944 -> Allow;
|
||||
| _ -> Drop;
|
||||
};
|
||||
```
|
||||
|
||||
### 6. NAT Policy
|
||||
|
||||
```fwl
|
||||
policy nat_prerouting : Frame
|
||||
on { hook = Prerouting, table = NAT, priority = DstNat }
|
||||
= {
|
||||
| Frame(_, IPv4(ip, _)) ->
|
||||
if perform FIB.daddrLocal(ip.dst)
|
||||
then DNATMap(forwards)
|
||||
else Allow;
|
||||
| _ -> Allow;
|
||||
};
|
||||
|
||||
policy nat_postrouting : Frame
|
||||
on { hook = Postrouting, table = NAT, priority = SrcNat }
|
||||
= {
|
||||
| Frame(_ -> wan, IPv4(ip, _)) if ip.src in rfc1918 -> Masquerade;
|
||||
| _ -> Allow;
|
||||
};
|
||||
```
|
||||
|
||||
### 7. Forward Policy calling a Rule
|
||||
|
||||
```fwl
|
||||
policy forward : Frame
|
||||
on { hook = Forward, table = Filter, priority = Filter }
|
||||
= {
|
||||
| _ if ct.state in { Established, Related } -> Allow;
|
||||
| frame if iif in lan_zone && oif == wan -> blockOutboundWG(frame);
|
||||
| _ if ct.status == DNAT -> Allow;
|
||||
| Frame(iif in lan_zone -> wan, _) -> Allow;
|
||||
| Frame(iif in lan_zone -> lan_zone, _) -> Allow;
|
||||
| Frame(wan -> lan_zone, IPv4(ip, TCP(tcp, _)))
|
||||
if (ip.dst, tcp.dport) in forwards -> Allow;
|
||||
| _ -> Drop;
|
||||
};
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## Haskell AST Sketch
|
||||
|
||||
The following gives the direct mapping from grammar to Haskell types.
|
||||
|
||||
```haskell
|
||||
-- src/FWL/AST.hs
|
||||
|
||||
data Program = Program [Config] [Decl]
|
||||
|
||||
data Config = Config { configTable :: Maybe String }
|
||||
|
||||
data Decl
|
||||
= DInterface Name IfaceKind [IfaceProp]
|
||||
| DZone Name [Name]
|
||||
| DImport Name Type String
|
||||
| DLet Name Type Expr
|
||||
| DPattern Name Type PacketPat
|
||||
| DFlow Name FlowExpr
|
||||
| DRule Name Type Expr -- expr must be LamExpr
|
||||
| DPolicy Name Type PolicyMeta ArmBlock
|
||||
|
||||
data PolicyMeta = PolicyMeta
|
||||
{ pmHook :: Hook
|
||||
, pmTable :: TableName
|
||||
, pmPriority :: Priority }
|
||||
|
||||
data Hook = Input | Forward | Output | Prerouting | Postrouting
|
||||
data TableName= Filter | NAT
|
||||
data Priority = PFilter | PDstNat | PSrcNat | PMangle | PInt Int
|
||||
|
||||
data IfaceKind = WAN | LAN | WireGuard | IKUser Name
|
||||
data IfaceProp = IPDynamic | IPCidr4 [CIDR] | IPCidr6 [CIDR]
|
||||
|
||||
-- Patterns
|
||||
|
||||
data Pat
|
||||
= PWild
|
||||
| PVar Name
|
||||
| PNamed Name -- named pattern reference (first-class)
|
||||
| PCtor Name [Pat]
|
||||
| PRecord Name [FieldPat]
|
||||
| PTuple [Pat]
|
||||
| PFrame (Maybe PathPat) Pat
|
||||
| PBytes [ByteElem]
|
||||
|
||||
data FieldPat
|
||||
= FPEq Name Literal
|
||||
| FPBind Name -- bind field to same-named var
|
||||
| FPAs Name Name -- bind field to fresh var
|
||||
|
||||
data PathPat = PathPat (Maybe EndpointPat) (Maybe EndpointPat)
|
||||
data EndpointPat
|
||||
= EPWild
|
||||
| EPName Name
|
||||
| EPMember Name Name -- iif `in` zoneName
|
||||
|
||||
data ByteElem = BEHex Word8 | BEWild | BEWildStar
|
||||
|
||||
-- Flow
|
||||
|
||||
data FlowExpr
|
||||
= FAtom Name
|
||||
| FSeq FlowExpr FlowExpr (Maybe Duration)
|
||||
|
||||
type Duration = (Int, TimeUnit)
|
||||
data TimeUnit = Seconds | Millis | Minutes | Hours
|
||||
|
||||
-- Types
|
||||
|
||||
data Type
|
||||
= TName Name [Type]
|
||||
| TTuple [Type]
|
||||
| TFun Type Type
|
||||
| TEffect [Name] Type
|
||||
|
||||
-- Expressions
|
||||
|
||||
data Expr
|
||||
= EVar Name
|
||||
| EQual [Name]
|
||||
| ELit Literal
|
||||
| ELam Name Expr
|
||||
| EApp Expr Expr
|
||||
| ECase Expr ArmBlock
|
||||
| EIf Expr Expr Expr
|
||||
| EDo [DoStmt]
|
||||
| ELet Name Expr Expr
|
||||
| ETuple [Expr]
|
||||
| ESet [Expr]
|
||||
| EMap [(Expr, Expr)]
|
||||
| EPerform [Name] [Expr] -- perform QualName(args)
|
||||
| EInfix InfixOp Expr Expr
|
||||
| EPrefix PrefixOp Expr
|
||||
|
||||
data InfixOp
|
||||
= OpAnd | OpOr | OpEq | OpNeq | OpLt | OpLte | OpGt | OpGte
|
||||
| OpIn | OpConcat | OpThen | OpBind
|
||||
|
||||
data PrefixOp = OpNot
|
||||
|
||||
data DoStmt
|
||||
= DSBind Name Expr
|
||||
| DSExpr Expr
|
||||
|
||||
type ArmBlock = [Arm]
|
||||
data Arm = Arm Pat (Maybe Expr) Expr -- pattern, guard, body
|
||||
|
||||
-- Actions (constructors of the Action type; parsed as Expr constructors)
|
||||
-- Allow | Drop | Continue | Masquerade
|
||||
-- | DNAT Expr | DNATMap Expr | Log LogLevel Expr
|
||||
|
||||
data LogLevel = LInfo | LWarn | LError
|
||||
|
||||
-- Literals
|
||||
|
||||
data Literal
|
||||
= LInt Int
|
||||
| LString String
|
||||
| LBool Bool
|
||||
| LIPv4 (Word8,Word8,Word8,Word8)
|
||||
| LIPv6 [Word16]
|
||||
| LCIDR Literal Int
|
||||
| LPort Int
|
||||
| LDuration Int TimeUnit
|
||||
| LHex Word8
|
||||
|
||||
type Name = String
|
||||
type CIDR = (Literal, Int)
|
||||
```
|
||||
Reference in New Issue
Block a user