# be careful: space between symbols

S := PROGRAM

PROGRAM := STATEMENTS

STATEMENTS := NONE_EMPTY_STATEMENTS 
    | EPSILON

NONE_EMPTY_STATEMENTS := STATEMENT 
    | STATEMENT ; STATEMENTS

STATEMENT := EXP 
    | LET_EXPRESSION 
    | IMPORT_EXPRESSION

# import module
IMPORT_EXPRESSION := import string as variable

# let statement
LET_EXPRESSION := let VARIABLE_DEFINITION_LIST

VARIABLE_DEFINITION_LIST := VARIABLE_DEFINITION 
    |  VARIABLE_DEFINITION , VARIABLE_DEFINITION_LIST

VARIABLE_DEFINITION := variable 
    | variable = EXP

EXP := JSON 
    | variable 
    | ABSTRACTION 
    | APPLICATION
    | CODE_BLOCK_EXP
    | CONDITION_EXP

CODE_BLOCK_EXP := { NONE_EMPTY_STATEMENTS }

CONDITION_EXP := EXP ? EXP : EXP

# standard math style function calling, eg: f(1, 2)
APPLICATION := CALLER ( ) 
    | CALLER ( EXP_LIST )

# CALLER stands for any thing which can used directly as a function
CALLER := variable 
    | APPLICATION 
    | ( ABSTRACTION )

EXP_LIST := EXP 
    | EXP , EXP_LIST

# define abstraction
ABSTRACTION := GUARDED_ABSTRACTION | GUARDED_ABSTRACTION verticalBar ABSTRACTION

GUARDED_ABSTRACTION := ABSTRACTION_BODY | ABSTRACTION_BODY, EXP_LIST

ABSTRACTION_BODY := ( ) -> EXP
    | ( LIST_VARIABLES ) -> EXP

LIST_VARIABLES := variable 
    | variable , LIST_VARIABLES

# below is the definition of ordinary json
JSON := string 
    | number 
    | OBJECT 
    | ARRAY 
    | true 
    | false 
    | null

OBJECT := { KEY_VALUES } 
    | { }

KEY_VALUES := string : JSON 
    | string : JSON , KEY_VALUES

ARRAY := [ ] 
    | [ LIST_VALUES ]

LIST_VALUES := JSON 
    | JSON , LIST_VALUES