Expressions & Statements

Core AST node types for control flow, variables, and expressions in Lodol workflows.

Expressions & Statements

Lodol workflow programs are represented as a JSON AST. Every node has a kind discriminant that identifies its type. This page covers the fundamental statement and expression nodes used to build workflow logic.

Statements

Statements form the top-level body of a workflow program and do not produce values.

Let

Declare a new variable in the current scope, optionally initialising it with an expression.

Field	Type	Required	Description
`kind`	`"LetStmt"`	Yes	Node discriminant
`name`	string	Yes	Variable name
`init`	expression	No	Initial value expression; variable is `null` if omitted

{
  "kind": "LetStmt",
  "name": "greeting",
  "init": { "kind": "Literal", "value": "Hello, world!" }
}

Assign

Assign a new value to an existing variable.

Field	Type	Description
`kind`	`"AssignStmt"`	Node discriminant
`name`	string	Name of the variable to assign
`value`	expression	New value expression

{
  "kind": "AssignStmt",
  "name": "count",
  "value": {
    "kind": "BinaryExpr",
    "op": "+",
    "left": { "kind": "VarRef", "name": "count" },
    "right": { "kind": "Literal", "value": 1 }
  }
}

If

Conditionally execute a branch. An optional elseBranch runs when the condition is falsy.

Field	Type	Required	Description
`kind`	`"IfStmt"`	Yes	Node discriminant
`cond`	expression	Yes	Condition — any expression; truthy/falsy semantics
`thenBranch`	statement	Yes	Statement executed when `cond` is truthy
`elseBranch`	statement	No	Statement executed when `cond` is falsy

{
  "kind": "IfStmt",
  "cond": {
    "kind": "BinaryExpr",
    "op": ">",
    "left": { "kind": "VarRef", "name": "score" },
    "right": { "kind": "Literal", "value": 90 }
  },
  "thenBranch": {
    "kind": "ExprStmt",
    "expr": { "kind": "Literal", "value": "pass" }
  },
  "elseBranch": {
    "kind": "ExprStmt",
    "expr": { "kind": "Literal", "value": "fail" }
  }
}

Expressions

Expressions are evaluated to produce a value. They can appear as the init of a LetStmt, the value of an AssignStmt, the cond of an IfStmt, arguments to built-ins, and anywhere else a value is expected.

Literal

A constant JSON value embedded directly in the AST.

Field	Type	Description
`kind`	`"Literal"`	Node discriminant
`value`	any	A JSON-compatible value: `number`, `string`, `boolean`, `null`, `array`, or `object`

{ "kind": "Literal", "value": 42 }

{ "kind": "Literal", "value": "hello" }

{ "kind": "Literal", "value": true }

{ "kind": "Literal", "value": null }

{ "kind": "Literal", "value": [1, 2, 3] }

{ "kind": "Literal", "value": { "key": "value" } }

Var Ref

Reference the current value of a variable by name.

Field	Type	Description
`kind`	`"VarRef"`	Node discriminant
`name`	string	Name of the variable to read

{ "kind": "VarRef", "name": "username" }

Member Expr

Read a named property from an object.

Field	Type	Description
`kind`	`"MemberExpr"`	Node discriminant
`obj`	expression	Object expression to access
`key`	string	Property name to read

{
  "kind": "MemberExpr",
  "obj": { "kind": "VarRef", "name": "user" },
  "key": "email"
}

Can be chained to reach nested properties:

{
  "kind": "MemberExpr",
  "obj": {
    "kind": "MemberExpr",
    "obj": { "kind": "VarRef", "name": "response" },
    "key": "data"
  },
  "key": "id"
}

Index Expr

Read an element from an array by a computed index expression.

Field	Type	Description
`kind`	`"IndexExpr"`	Node discriminant
`arr`	expression	Array expression to index into
`idx`	expression	Index expression (should evaluate to a non-negative integer)

{
  "kind": "IndexExpr",
  "arr": { "kind": "VarRef", "name": "items" },
  "idx": { "kind": "Literal", "value": 0 }
}

Unary Expr

Apply a unary operator to a single operand.

Field	Type	Description
`kind`	`"UnaryExpr"`	Node discriminant
`op`	`"-"` \| `"+"` \| `"!"`	Operator
`right`	expression	Operand

Operator	Description	Example
`-`	Numeric negation	`-x`
`+`	Numeric identity (no-op)	`+x`
`!`	Logical NOT	`!isActive`

{
  "kind": "UnaryExpr",
  "op": "!",
  "right": { "kind": "VarRef", "name": "isActive" }
}

Binary Expr

Apply a binary operator to two operands.

Field	Type	Description
`kind`	`"BinaryExpr"`	Node discriminant
`op`	string	Operator (see table below)
`left`	expression	Left operand
`right`	expression	Right operand

Arithmetic operators

Operator	Description
`+`	Addition (numbers) or string concatenation
`-`	Subtraction
`*`	Multiplication
`/`	Division

Comparison operators

Operator	Description
`==`	Deep equality
`!=`	Deep inequality
`<`	Less than
`<=`	Less than or equal
`>`	Greater than
`>=`	Greater than or equal

Logical operators

Operator	Description
`&&`	Logical AND (short-circuits)
`\|\|`	Logical OR (short-circuits)

{
  "kind": "BinaryExpr",
  "op": "&&",
  "left": {
    "kind": "BinaryExpr",
    "op": ">=",
    "left": { "kind": "VarRef", "name": "age" },
    "right": { "kind": "Literal", "value": 18 }
  },
  "right": {
    "kind": "BinaryExpr",
    "op": "==",
    "left": { "kind": "VarRef", "name": "verified" },
    "right": { "kind": "Literal", "value": true }
  }
}

Expressions & Statements

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