Implementation Details

While the user writes Waves Contracts code in the high-level language, Waves Contracts execution engine is a straightforward evaluator of low-level expression tree within context. In order to achieve that, there're several stages which make text script produce an execution result. These are:

  1. Parsing
  2. Compilation (typechecking & compiling).
  3. Deserialization
  4. Cost computation
  5. Evaluation

1. Parsing Stage

The Parser builds untyped Abstract Syntax Tree(AST) from the script text. At this phase, Only syntax rules are checked such as correct variable names, function invocation with () and so on.

The syntax doesn't require line breaks (\n) or ;. The full description goes as follows:

number = [+-]?['0'-'9']+
string = """, [1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-]*, """
byteVector = "base58'", [123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz]* , "'"
binaryOp = expr , ("||" | "&&" | "==" | "!=" | ">=" | ">" | "<" | "<=" | "+" | "-" | "*" | "%" | "/") , expr
unaryOp =  ("-" | "!") , expr
expression = binaryOp | atom
varName = {latin-numeric string strating with char, excluding keywords}
let = "let " , varName , "=" , block
block = let? , expr
if = "if" , "(" , block , ")" , "then" , block , "else" , block
ref = varName
getter = ref , "." , varName
functionCall = varName , "(" , expr* , ")"
listAccess = expr , "[" , expr* , "]"
braces = "(" , block , ")"
curlyBraces = "{" , block , "}"
atom = if | functionCall | listAccess | byteVector | string | number | braces | curlyBraces | getter | ref

All available types are:

Types Description
Bottom Type NOTHING
Primitive Types UNIT, LONG, BYTEVECTOR, BOOLEAN, STRING
Complex Types TYPEREF(typename), Option(Type) and List(Type)

Note. User can't create new types, only predefined ones are available.

The Parser generates AST based on the following constructs:

Constract Description
LET(name, expr) Used to define a variable
REF(name) Used to access its value
GETTER(expr, fieldName) Used to access field of structure
FUNCTION_CALL(name, argExprs) Used to invoke a predefined function within context
IF(clause, ifTrue, ifFalse) Used for lazy branching
CONST_LONG(long), CONST_BYTEVECTOR(byteVector), CONST_STRING(string) Used as Leafs
BINARY_OP(EXPR, OP_KIND, EXPR) Used exclusively for ease of parsing

2. Compilation Stage

The Untyped AST is enriched with types and then they're checked according to the function signatures.

It operates within a context of type definitions, types of defined values and predefined function signatures.

An expression operates the base type EXPR, and its sub-type of BLOCK. Each EXPR has a type and is one of:

Types Description
LET(name, block) Used to define a variable
GETTER(expr, fieldName) Used to access field of structure
FUNCTION_CALL(name, argBlocks) Used to invoke a predefined function within context
IF(clause, ifTrueBlock, ifFalseBlock) Used for Lazy branching
CONST_LONG(long), CONST_BYTEVECTOR(byteVector), CONST_STRING(string), REF(name) Used as Leafs

This set doesn't include BINARY_OP as well, it gets translated to FUNCTION_CALL. This set doesn’t include constructs that are used exclusively for ease of parsing. The pattern matching mechanism is replaced by IF + .isInstanceOf call.

This step is important to validate user input so that less mistakes are made: for instance, 3 + false is valid syntactically, but typechecker won't compile it, because + function requires two arguments of type Long.

The result types of each EXPR aren't shown to the outside and don't go to the next stage.

Note. The output of this stage is exactly what is sent to the blockchain.

3. Deserialization Stage

In this stage RIDE only checks syntax rules, like correct variable names, function invocation with () and so on.

4. Cost Computation Stage

It operates within a context of type definitions, types of defined values and predefined function signatures.

An expression operates the base type EXPR, and its sub-type BLOCK. Each EXPR has a type and this type is one of:

  • LET(name, block) to define a variable.
  • GETTER(expr, fieldName) to access field of structure.
  • FUNCTION_CALL(name, argBlocks) to invoke a predefined function within context
  • IF(clause, ifTrueBlock, ifFalseBlock) for lazy branching.
  • Leafs:CONST_LONG(long), CONST_BYTEVECTOR(byteVector), CONST_STRING(string), REF(name), TRUE, FALSE

5. Evaluator Stage

Evaluator operates a typed expression tree within a context. It traverses the low-level typed AST (which is produced at the previous step) and return either the execution result or an execution error.

Context contains:

  • A map of predefined functions with implementation.
  • Predefined types, e.g. structures.
  • Lazy values, that can be calculated upon calls within given tree path, cannot be re-defined and they are calculated maximum once.

If the evaluator has exception results(for instance, Long Overflow), the script evaluation will be false.

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