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Version: v1.12


Expr is a stack-based virtual machine. It compiles expressions to bytecode and runs it. The compilation is done in a few steps:

  • Parse expression to AST
  • Type check AST
    • Apply operator overloading
    • Apply custom AST patching
  • Optimize AST
  • Compile AST to a bytecode

The compiler has a bunch of optimization which will produce a more optimal program.

In array

value in ['foo', 'bar', 'baz']

If Expr finds an in or not in expression with an array, it will be transformed into:

value in {"foo": true, "bar": true, "baz": true}

Constant folding

Arithmetic expressions with constants are computed on compile step and replaced with the result.

-(2 - 5) ** 3 - 2 / (+4 - 3) + -2

Will be compiled into just a single number:


In range

user.Age in 18..32

Will be replaced with a binary operator:

18 <= user.Age && user.Age <= 32

Const range


Ranges computed on the compile stage, are replaced with pre-created slices.

Const expr

If some function is marked as a constant expression with expr.ConstExpr. It will be replaced with the result of the call if all arguments are constants.


Will be replaced with the result of fib(42)` on the compile step.

ConstExpr Example