code: func interpolationFuncSha256() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			s := args[0].(string)
  			h := sha256.New()
  			h.Write([]byte(s))
  			hash := hex.EncodeToString(h.Sum(nil))
  			return hash, nil
  		},
  	}
  }
docs: // hexadecimal representation of sha256 sum