Specification: BQN grammar

BQN's grammar is given below. Terms are defined in a BNF variant. However, handling special names properly is possible but difficult in BNF, so they are explained in text along with the block grammar.

The symbols s, F, _m, and _c_ are identifier tokens with subject, function, 1-modifier, and 2-modifier classes respectively. Similarly, sl, Fl, _ml, and _cl_ refer to literals and primitives of those classes. While names in the BNF here follow the identifier naming scheme, this is informative only: syntactic roles are no longer used after parsing and cannot be inspected in a running program.

A program is a list of statements. Almost all statements are expressions. Namespace export statements, and valueless results stemming from ·, or 𝕨 in a monadic block function, can be used as statements but not expressions.

PROGRAM  = ⋄? ( STMT ⋄ )* STMT ⋄?
STMT     = EXPR | nothing | EXPORT
⋄        = ( "⋄" | "," | LF | CR )+
EXPR     = subExpr | FuncExpr | _m1Expr | _m2Expr_
EXPORT   = LHS_ELT? "⇐"

Here we define the "atomic" forms of functions and modifiers, which are either single tokens or enclosed in paired symbols. Stranded lists with ‿, which binds more tightly than any form of execution, are also included.

ANY      = atom | Func | _mod1 | _mod2_
_mod2_   = ( atom "." )? _c_ | _cl_ | "(" _m2Expr_ ")" | _blMod2_
_mod1    = ( atom "." )? _m  | _ml  | "(" _m1Expr  ")" | _blMod1
Func     = ( atom "." )?  F  |  Fl  | "(" FuncExpr ")" |  BlFunc
atom     = ( atom "." )?  s  |  sl  | "(" subExpr  ")" |  blSub | array
array    = "⟨" ⋄? ( ( EXPR ⋄ )* EXPR ⋄? )? "⟩"
         | "[" ⋄?   ( EXPR ⋄ )* EXPR ⋄?    "]"
subject  = atom | ANY ( "‿" ANY )+

Starting at the highest-order objects, modifiers have simple syntax. In most cases the syntax for ← and ↩ is the same, but only ↩ can be used for modified assignment. The export arrow ⇐ can be used in the same ways as ←, but it can also be used at the beginning of a header to force a namespace result, or with no expression on the right in an EXPORT statement.

ASGN     = "←" | "⇐" | "↩"
_m2Expr_ = _mod2_
         | _c_ ASGN _m2Expr_
_m1Expr  = _mod1
         | _m  ASGN _m1Expr

Functions can be formed by applying modifiers, or with trains. Modifiers are left-associative, so that the left operand (Operand) can include modifier applications but the right operand (subject | Func) cannot. Trains are right-associative, but bind less tightly than modifiers. Assignment is not allowed in the top level of a train: it must be parenthesized.

Derv     = Func
         | Operand _mod1
         | Operand _mod2_ ( subject | Func )
Operand  = subject
         | Derv
Fork     = Derv
         | Operand Derv Fork          # 3-train
         | nothing Derv Fork          # 2-train
Train    = Fork
         | Derv Fork                  # 2-train
FuncExpr = Train
         | F ASGN FuncExpr

Subject expressions consist mainly of function application. We also define nothing-statements, which have very similar syntax to subject expressions but do not permit assignment. They can be used as an STMT or in place of a left argument.

arg      = subject
         | ( subject | nothing )? Derv subExpr
nothing  = "·"
         | ( subject | nothing )? Derv nothing
subExpr  = arg
         | lhs ASGN subExpr
         | lhs Derv "↩" subExpr?      # Modified assignment

The target of subject assignment can be compound to allow for destructuring. List and namespace assignment share the nodes lhsList and lhsStr and cannot be completely distinguished until execution. The term sl in LHS_SUB is used for header inputs below: as an additional rule, it cannot be used in the lhs term of a subExpr node.

NAME     = s | F | _m | _c_
LHS_SUB  = "·" | lhsList | lhsArray | sl
LHS_ANY  = NAME | LHS_SUB | "(" LHS_ELT ")"
LHS_ATOM = LHS_ANY | "(" lhsStr ")"
LHS_ELT  = LHS_ANY | lhsStr
LHS_ENTRY= LHS_ELT | lhs "⇐" NAME
lhsStr   = LHS_ATOM ( "‿" LHS_ATOM )+
lhsList  = "⟨" ⋄? ( ( LHS_ENTRY ⋄ )* LHS_ENTRY ⋄? )? "⟩"
lhsArray = "[" ⋄? ( ( LHS_ELT   ⋄ )* LHS_ELT   ⋄? )? "]"
lhsComp  = LHS_SUB | lhsStr | "(" lhs ")"
lhs      = s | lhsComp

A header looks like a name for the thing being headed, or its application to inputs (possibly twice in the case of modifiers). As with assignment, it is restricted to a simple form with no extra parentheses. The full list syntax is allowed for arguments. A plain name is called a label and can be used for a block with or without arguments. First we define headers IMM_HEAD that include no arguments.

headW    = lhs | "𝕨"
headX    = lhs | "𝕩"
HeadF    = lhs | F | "𝕗" | "𝔽"
HeadG    = lhs | F | "𝕘" | "𝔾"
FuncLab  = F | "𝕊"
Mod1Lab  = _m  | "_𝕣"
Mod2Lab  = _c_ | "_𝕣_"
FuncName = FuncLab
Mod1Name = HeadF Mod1Lab
Mod2Name = HeadF Mod2Lab HeadG
LABEL    =         FuncLab  | Mod1Lab  | Mod2Lab
IMM_HEAD = LABEL | FuncName | Mod1Name | Mod2Name

There are some extra possibilities for a header that specifies arguments. As a special rule, a monadic function header specifically can omit the function when the argument is not just a name (as this would conflict with a subject label). Additionally, an inference header doesn't affect evaluation of the function, but describes how an inferred property (Undo) should be computed. Here "˜" and "⁼" are both specific instances of the _ml token.

ARG_HEAD = LABEL
         | headW? IMM_HEAD      "⁼"? headX
         | headW  IMM_HEAD "˜"  "⁼"  headX
         |        FuncName "˜"? "⁼"
         | lhsComp

A block is written with braces. It contains bodies, which are lists of statements, separated by semicolons. Multiple bodies can handle different cases, as determined by headers and predicates. A header is written before its body with a separating colon, and an expression other than the last in a body can be made into a predicate by following it with the separator-like ?.

An I_CASE, A_CASE, or S_CASE is called a general case or general body if it has no header or predicate, or, more formally, it doesn't directly include a ":" token and its BODY node doesn't use the EXPR ⋄? "?" ⋄? case. A program must satisfy some additional rules regarding general cases, but these are not needed to resolve the grammar and shouldn't strictly be considered part of it. First, no general body can appear before a body that isn't general in a block. Second, a IMM_BLK or blSub can directly contain at most one general body and an ARG_BLK at most two (these are monadic and dyadic cases).

BODY     = ⋄? ( STMT ⋄ | EXPR ⋄? "?" ⋄? )* STMT ⋄?
I_CASE   = ( ⋄? IMM_HEAD ⋄? ":" )? BODY
A_CASE   = ( ⋄? ARG_HEAD ⋄? ":" )? BODY
S_CASE   = ( ⋄? s        ⋄? ":" )? BODY
IMM_BLK  = "{" ( I_CASE ";" )* I_CASE "}"
ARG_BLK  = "{" ( A_CASE ";" )* A_CASE "}"
blSub    = "{" ( S_CASE ";" )* S_CASE "}"
BlFunc   =           ARG_BLK
_blMod1  = IMM_BLK | ARG_BLK
_blMod2_ = IMM_BLK | ARG_BLK

Three additional rules apply to blocks, allowing the ambiguous grammar above to be disambiguated. They are shown in the table below. First, each block type allows the special names in its row to be used as the given token types within BODY terms (not headers). Except for the spaces labelled "None", each of these four columns is cumulative, so that a given entry also includes all the entries above it. Second, a block can't contain one of the tokens from the "label" column of a different row. Third, each BlFunc, _blMod1, and _blMod2_ term must contain one of the names on, and not above, the corresponding row (including the "label" column).

The rules for special names can be expressed in BNF by making many copies of all expression rules above. For each "level", or row in the table, a new version of every rule should be made that allows that level but not higher ones, and another version should be made that requires exactly that level. The values themselves should be included in s, F, _m, and _c_ for these copies. Then the "allowed" rules are made simply by replacing the terms they contain (excluding blSub and so on) with the same "allowed" versions, and "required" rules are constructed using both "allowed" and "required" rules. For every part of a production rule, an alternative should be created that requires the relevant name in that part while allowing it in the others. For example, the definition of arg as subject | ( subject | nothing )? Derv subExpr would be transformed to

arg_req1 = subject_req1
         | ( subject_req1 | nothing_req1 ) Derv_allow1 subExpr_allow1
         | ( subject_allow1 | nothing_allow1 )? Derv_req1 subExpr_allow1
         | ( subject_allow1 | nothing_allow1 )? Derv_allow1 subExpr_req1

Quite tedious. The explosion of rules is partly due to the fact that the block-typing rule falls into a weaker class of grammars than the other rules. Most of BQN is deterministic context-free but block-typing is not, only context-free. Fortunately block typing does not introduce the parsing difficulties that can be present in a general context-free grammar, and it can easily be performed in linear time: after scanning but before parsing, move through the source code maintaining a stack of the current top-level set of braces. Whenever a colon or special name is encountered, annotate that set of braces to indicate that it is present. When a closing brace is encountered and the top brace is popped off the stack, the type is needed if there was no colon, and can be found based on which names were present. One way to present this information to the parser is to replace the brace tokens with new tokens that indicate the type.