The AST in Nim ================= This section describes how the AST is modelled with Nim's type system. The AST consists of nodes (``PNimrodNode``) with a variable number of children. Each node has a field named ``kind`` which describes what the node contains: .. code-block:: nim type TNimrodNodeKind = enum ## kind of a node; only explanatory nnkNone, ## invalid node kind nnkEmpty, ## empty node nnkIdent, ## node contains an identifier nnkIntLit, ## node contains an int literal (example: 10) nnkStrLit, ## node contains a string literal (example: "abc") nnkNilLit, ## node contains a nil literal (example: nil) nnkCaseStmt, ## node represents a case statement ... ## many more PNimrodNode = ref TNimrodNode TNimrodNode {.final.} = object case kind: TNimrodNodeKind ## the node's kind of nnkNone, nnkEmpty, nnkNilLit: nil ## node contains no additional fields of nnkCharLit..nnkInt64Lit: intVal: biggestInt ## the int literal of nnkFloatLit..nnkFloat64Lit: floatVal: biggestFloat ## the float literal of nnkStrLit..nnkTripleStrLit: strVal: string ## the string literal of nnkIdent: ident: TNimrodIdent ## the identifier of nnkSym: symbol: PNimrodSymbol ## the symbol (after symbol lookup phase) else: sons: seq[PNimrodNode] ## the node's sons (or children) For the ``PNimrodNode`` type, the ``[]`` operator has been overloaded: ``n[i]`` is ``n``'s ``i``-th child. To specify the AST for the different Nim constructs, the notation ``nodekind(son1, son2, ...)`` or ``nodekind(value)`` or ``nodekind(field=value)`` is used. Some child may be missing. A missing child is a node of kind ``nnkEmpty``; a child can never be nil. Leaf nodes/Atoms ================ A leaf of the AST often corresponds to a terminal symbol in the concrete syntax. ----------------- --------------------------------------------- Nim expression corresponding AST ----------------- --------------------------------------------- ``42`` ``nnkIntLit(intVal = 42)`` ``42'i8`` ``nnkInt8Lit(intVal = 42)`` ``42'i16`` ``nnkInt16Lit(intVal = 42)`` ``42'i32`` ``nnkInt32Lit(intVal = 42)`` ``42'i64`` ``nnkInt64Lit(intVal = 42)`` ``42.0`` ``nnkFloatLit(floatVal = 42.0)`` ``42.0'f32`` ``nnkFloat32Lit(floatVal = 42.0)`` ``42.0'f64`` ``nnkFloat64Lit(floatVal = 42.0)`` ``"abc"`` ``nnkStrLit(strVal = "abc")`` ``r"abc"`` ``nnkRStrLit(strVal = "abc")`` ``"""abc"""`` ``nnkTripleStrLit(strVal = "abc")`` ``' '`` ``nnkCharLit(intVal = 32)`` ``nil`` ``nnkNilLit()`` ``myIdentifier`` ``nnkIdent(ident = !"myIdentifier")`` ``myIdentifier`` after lookup pass: ``nnkSym(symbol = ...)`` ----------------- --------------------------------------------- Identifiers are ``nnkIdent`` nodes. After the name lookup pass these nodes get transferred into ``nnkSym`` nodes. However, a macro receives an AST that has not been checked for semantics and thus the identifiers have not been looked up. Macros should deal with ``nnkIdent`` nodes and do not need to deal with ``nnkSym`` nodes. Calls/expressions ================= Command call ------------ Concrete syntax: .. code-block:: nim echo "abc", "xyz" AST: .. code-block:: nim nnkCommand(nnkIdent(!"echo"), nnkStrLit("abc"), nnkStrLit("xyz")) Call with ``()`` ---------------- Concrete syntax: .. code-block:: nim echo("abc", "xyz") AST: .. code-block:: nim nnkCall(nnkIdent(!"echo"), nnkStrLit("abc"), nnkStrLit("xyz")) Infix operator call ------------------- Concrete syntax: .. code-block:: nim "abc" & "xyz" AST: .. code-block:: nim nnkInfix(nnkIdent(!"&"), nnkStrLit("abc"), nnkStrLit("xyz")) Prefix operator call -------------------- Concrete syntax: .. code-block:: nim ? "xyz" AST: .. code-block:: nim nnkPrefix(nnkIdent(!"?"), nnkStrLit("abc")) Postfix operator call --------------------- **Note:** There are no postfix operators in Nim. However, the ``nnkPostfix`` node is used for the *asterisk export marker* ``*``: Concrete syntax: .. code-block:: nim identifier* AST: .. code-block:: nim nnkPostfix(nnkIdent(!"*"), nnkIdent(!"identifier")) Call with named arguments ------------------------- Concrete syntax: .. code-block:: nim writeln(file=stdout, "hallo") AST: .. code-block:: nim nnkCall(nnkIdent(!"writeln"), nnkExprEqExpr(nnkIdent(!"file"), nnkIdent(!"stdout")), nnkStrLit("hallo")) Dereference operator ``^`` -------------------------- Concrete syntax: .. code-block:: nim x^ AST: .. code-block:: nim nnkDerefExpr(nnkIdent(!"x")) Addr operator ------------- Concrete syntax: .. code-block:: nim addr(x) AST: .. code-block:: nim nnkAddr(nnkIdent(!"x")) Cast operator ------------- Concrete syntax: .. code-block:: nim cast[T](x) AST: .. code-block:: nim nnkCast(nnkIdent(!"T"), nnkIdent(!"x")) Object access operator ``.`` ---------------------------- Concrete syntax: .. code-block:: nim x.y AST: .. code-block:: nim nnkDotExpr(nnkIdent(!"x"), nnkIdent(!"y")) Array access operator ``[]`` ---------------------------- Concrete syntax: .. code-block:: nim x[y] AST: .. code-block:: nim nnkBracketExpr(nnkIdent(!"x"), nnkIdent(!"y")) Parentheses ----------- Parentheses for affecting operator precedence or tuple construction are built with the ``nnkPar`` node. Concrete syntax: .. code-block:: nim (1, 2, (3)) AST: .. code-block:: nim nnkPar(nnkIntLit(1), nnkIntLit(2), nnkPar(nnkIntLit(3))) Curly braces ------------ Curly braces are used as the set constructor. Concrete syntax: .. code-block:: nim {1, 2, 3} AST: .. code-block:: nim nnkCurly(nnkIntLit(1), nnkIntLit(2), nnkIntLit(3)) Brackets -------- Brackets are used as the array constructor. Concrete syntax: .. code-block:: nim [1, 2, 3] AST: .. code-block:: nim nnkBracket(nnkIntLit(1), nnkIntLit(2), nnkIntLit(3)) Ranges ------ Ranges occur in set constructors, case statement branches or array slices. Concrete syntax: .. code-block:: nim 1..3 AST: .. code-block:: nim nnkRange(nnkIntLit(1), nnkIntLit(3)) If expression ------------- The representation of the if expression is subtle, but easy to traverse. Concrete syntax: .. code-block:: nim if cond1: expr1 elif cond2: expr2 else: expr3 AST: .. code-block:: nim nnkIfExpr( nnkElifExpr(cond1, expr1), nnkElifExpr(cond2, expr2), nnkElseExpr(expr3) ) Statements ========== If statement ------------ The representation of the if statement is subtle, but easy to traverse. If there is no ``else`` branch, no ``nnkElse`` child exists. Concrete syntax: .. code-block:: nim if cond1: stmt1 elif cond2: stmt2 elif cond3: stmt3 else: stmt4 AST: .. code-block:: nim nnkIfStmt( nnkElifBranch(cond1, stmt1), nnkElifBranch(cond2, stmt2), nnkElifBranch(cond3, stmt3), nnkElse(stmt4) ) When statement -------------- Like the ``if`` statement, but the root has the kind ``nnkWhenStmt``. Assignment ---------- Concrete syntax: .. code-block:: nim x = 42 AST: .. code-block:: nim nnkAsgn(nnkIdent(!"x"), nnkIntLit(42)) Statement list -------------- Concrete syntax: .. code-block:: nim stmt1 stmt2 stmt3 AST: .. code-block:: nim nnkStmtList(stmt1, stmt2, stmt3) Case statement -------------- Concrete syntax: .. code-block:: nim case expr1 of expr2, expr3..expr4: stmt1 of expr5: stmt2 elif cond1: stmt3 else: stmt4 AST: .. code-block:: nim nnkCaseStmt( expr1, nnkOfBranch(expr2, nnkRange(expr3, expr4), stmt1), nnkOfBranch(expr5, stmt2), nnkElifBranch(cond1, stmt3), nnkElse(stmt4) ) The ``nnkElifBranch`` and ``nnkElse`` parts may be missing. While statement --------------- Concrete syntax: .. code-block:: nim while expr1: stmt1 AST: .. code-block:: nim nnkWhileStmt(expr1, stmt1) For statement ------------- Concrete syntax: .. code-block:: nim for ident1, ident2 in expr1: stmt1 AST: .. code-block:: nim nnkForStmt(ident1, ident2, expr1, stmt1) Try statement ------------- Concrete syntax: .. code-block:: nim try: stmt1 except e1, e2: stmt2 except e3: stmt3 except: stmt4 finally: stmt5 AST: .. code-block:: nim nnkTryStmt( stmt1, nnkExceptBranch(e1, e2, stmt2), nnkExceptBranch(e3, stmt3), nnkExceptBranch(stmt4), nnkFinally(stmt5) ) Return statement ---------------- Concrete syntax: .. code-block:: nim return expr1 AST: .. code-block:: nim nnkReturnStmt(expr1) Yield statement --------------- Like ``return``, but with ``nnkYieldStmt`` kind. Discard statement ----------------- Like ``return``, but with ``nnkDiscardStmt`` kind. Continue statement ------------------ Concrete syntax: .. code-block:: nim continue AST: .. code-block:: nim nnkContinueStmt() Var section ----------- To be written. Const section ------------- To be written. Type section ------------ To be written. Procedure declaration --------------------- To be written. Iterator declaration -------------------- To be written. Template declaration -------------------- To be written. Macro declaration ----------------- To be written. Special node kinds ================== There are several node kinds that are used for semantic checking or code generation. These are accessible from this module, but should not be used. Other node kinds are especially designed to make AST manipulations easier. These are explained here. To be written.