Add EBNF for AST to book

book/src/contributing/ast_ebnf.md

@@ -0,0 +1,237 @@
+We currently take inputs from the following AST (visualization can be done
+using <https://rr.red-dove.com/ui>). Literals are strings, numbers and
+booleans
+
+``` ebnf
+char ::= [a-zA-Z]
+string ::= char*
+digit ::= [0-9]
+uint ::= digit+
+int ::= ("-")? uint
+float ::= int (".")? uint
+bool ::= "true" | "false"
+
+literal ::=
+| "\"" string "\""
+| "'" char "'"
+| int
+| float
+| bool
+```
+
+We support a number of simple types characters, strings, booleans and
+numbers. Number types for integers (8,16,32,64,128 bit or machine sized)
+and floats (16,32, or 64 bit). Composite types are tuples, fixed length
+lists (arrays), variable length lists (vectors/slices), ptr types, and
+function types. Lastly we have named types defined by items, e.g. enums
+and structs.
+
+``` ebnf
+generic_value ::=
+| "'" ident
+| ty
+| expr
+
+impl ::=
+| "self"
+| goal
+| "dyn"
+| <!TODO!>
+goal ::= ident "<" (generic_value ",")* ">"
+
+ty ::=
+| "bool"
+| "char"
+| "u8" | "u16" | "u32" | "u64" | "u128" | "usize"
+| "i8" | "i16" | "i32" | "i64" | "i128" | "isize"
+| "f16" | "f32" | "f64"
+| "str"
+| (ty ",")*
+| "[" ty ";" int "]"
+| "[" ty "]"
+| "*const" ty | "*mut" ty
+| "*" expr | "*mut" expr
+| ident
+| (ty "->")* ty
+| impl "::" ident
+| "impl" ty
+| (goal)*
+```
+
+The patterns allowed reflect these types. Wildcard patterns, literal
+types, typed patterns, list patterns, record or tuple patterns.
+
+``` ebnf
+pat ::=
+| "_"
+| pat ":" ty
+| (ident "{" (ident ":" pat ";")* "}" | ident "(" (pat ",")* ")" )
+| (pat "|")* pat
+| ("[" (expr ",")* "]" | "[" expr ";" int "]")
+| "&" pat
+| literal
+| ident "@" pat
+```
+
+The simple expressions are literals, local or global variables, type
+casts, assignments and lists. Control flow expressions, if statements,
+match statements, loops, return, break and continue. The rest is blocks,
+macro calls, lambda functions and borrowing.
+
+``` ebnf
+local_var ::= ident
+global_var ::= ident
+
+modifiers ::= "" | "unsafe" modifiers | "const" modifiers
+
+lhs ::= pat
+param ::= pat
+
+expr ::=
+| "if" expr "{" expr "}" ("else" "{" expr "}")?
+| expr "(" (expr ",")* ")"
+| literal
+| "[" (expr ",")* "]" | "[" expr ";" int "]"
+| ident "(" (expr ",")* ")" | ident "{" (ident ":"expr ";")* "}" | ident "{" (ident ":"expr ";")* ".." expr "}"
+| "match" expr "{" (("|" pat)* "=>" (expr "," | "{" expr "}"))* "}"
+| "let" pat (":" ty)? ":=" expr ";" expr
+| modifiers "{" expr "}"
+| local_var
+| global_var
+| expr "as" ty
+| loops_expr
+| lhs "=" expr
+| "return" expr
+| expr "?"
+| "&" ("mut")? expr
+| "&" expr "as" "&const _" | "&mut" expr "as" "&mut _"
+| "|" param "|" expr
+
+loops_expr ::=
+| "loop" "{" expr "}"
+| "while" "(" expr ")" "{" expr "}"
+| "for" "(" pat "in" expr ")" "{" expr "}"
+| "for" "(" "let" ident "in" expr ".." expr ")" "{" expr "}"
+| "break" expr
+| "continue"
+```
+
+The items supported are functions, type aliasing, enums, structs, trait
+definitions and implementations, and imports.
+
+``` ebnf
+item ::=
+| modifiers "fn" ident "(" (pat ":" ty ",")* ")" (":" ty)? "{" expr "}"
+| "type" ident "=" ty
+| "enum" ident "=" "{" (ident ("(" (ty)* ")")? ",")* "}"
+| "struct" ident "=" "{" (ident ":" ty ",")* "}"
+| "trait" ident "{" (trait_item)* "}"
+| "impl" ("<" (generics ",")* ">")? ident "for" ty "{" (impl_item)* "}"
+| "use" path ";"
+```
+
+The full AST description
+
+``` ebnf
+char ::= [a-zA-Z]
+string ::= char*
+digit ::= [0-9]
+uint ::= digit+
+int ::= ("-")? uint
+float ::= int (".")? uint
+bool ::= "true" | "false"
+
+literal ::=
+| "\"" string "\""
+| "'" char "'"
+| int
+| float
+| bool
+
+ident ::= string
+
+generic_value ::=
+| "'" ident
+| ty
+| expr
+
+impl ::=
+| "self"
+| goal
+| "dyn"
+| <!TODO!>
+goal ::= ident "<" (generic_value ",")* ">" 
+
+ty ::=
+| "bool"
+| "char"
+| "u8" | "u16" | "u32" | "u64" | "u128" | "usize"
+| "f16" | "f32" | "f64"
+| "str"
+| (ty ",")*
+| ident "(" (ident ",")* ")"
+| "[" ty ";" int "]"
+| "[" ty "]"
+| "*const" ty | "*mut" ty
+| "*" expr | "*mut" expr
+| ident
+| (ty "->")* ty
+| impl "::" ident
+| "impl" ty
+| "dyn" (goal)*
+
+pat ::=
+| "_"
+| pat ":"
+| (ident "{" (ident ":" pat ";")* "}" | ident "(" (pat ",")* ")" )
+| (pat "|")* pat
+| ("[" (expr ",")* "]" | "[" expr ";" int "]")
+| "&" pat
+| literal
+| ident "@" pat
+
+local_var ::= ident
+global_var ::= ident
+
+modifiers ::= "" | "unsafe" modifiers | "const" modifiers
+
+lhs ::= pat
+param ::= pat
+
+expr ::=
+| "if" expr "{" expr "}" ("else" "{" expr "}")?
+| expr "(" (expr ",")* ")"
+| literal
+| "[" (expr ",")* "]" | "[" expr ";" int "]"
+| ident "(" (expr ",")* ")" | ident "{" (ident ":"expr ";")* "}" | ident "{" (ident ":"expr ";")* ".." expr "}"
+| "match" expr "{" (("|" pat)* "=>" (expr "," | "{" expr "}"))* "}"
+| "let" pat (":" ty)? ":=" expr ";" expr
+| modifiers "{" expr "}"
+| local_var
+| global_var
+| expr "as" ty
+| loops_expr
+| lhs "=" expr
+| "return" expr
+| expr "?"
+| "&" ("mut")? expr
+| "&" expr "as" "&const _" | "&mut" expr "as" "&mut _"
+| "|" param "|" expr
+
+loops_expr ::=
+| "loop" "{" expr "}"
+| "while" "(" expr ")" "{" expr "}"
+| "for" "(" pat "in" expr ")" "{" expr "}"
+| "for" "(" "let" ident "in" expr ".." expr ")" "{" expr "}"
+| "break" expr
+| "continue"
+
+item ::=
+| modifiers "fn" ident "(" (pat ":" ty ",")* ")" (":" ty)? "{" expr "}"
+| "type" ident "=" ty
+| "enum" ident "=" "{" (ident ("(" (ty)* ")")? ",")* "}"
+| "struct" ident "=" "{" (ident ":" ty ",")* "}"
+| "trait" ident "{" (trait_item)* "}"
+| "impl" ("<" (generics ",")* ">")? ident "for" ty "{" (impl_item)* "}"
+| "use" path ";"
+```