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====================== Nim Tutorial (Part II) ====================== :Author: Andreas Rumpf :Version: |nimversion| .. contents:: Introduction ============ "Repetition renders the ridiculous reasonable." -- Norman Wildberger This document is a tutorial for the advanced constructs of the *Nim* programming language. **Note that this document is somewhat obsolete as the** `manual <manual.html>`_ **contains many more examples of the advanced language features.** Pragmas ======= Pragmas are Nim's method to give the compiler additional information/ commands without introducing a massive number of new keywords. Pragmas are enclosed in the special ``{.`` and ``.}`` curly dot brackets. This tutorial does not cover pragmas. See the `manual <manual.html#pragmas>`_ or `user guide <nimc.html#additional-features>`_ for a description of the available pragmas. Object Oriented Programming =========================== While Nim's support for object oriented programming (OOP) is minimalistic, powerful OOP techniques can be used. OOP is seen as *one* way to design a program, not *the only* way. Often a procedural approach leads to simpler and more efficient code. In particular, prefering composition over inheritance is often the better design. Objects ------- Like tuples, objects are a means to pack different values together in a structured way. However, objects provide many features that tuples do not: They provide inheritance and information hiding. Because objects encapsulate data, the ``T()`` object constructor should only be used internally and the programmer should provide a proc to initialize the object (this is called a *constructor*). Objects have access to their type at runtime. There is an ``of`` operator that can be used to check the object's type: .. code-block:: nim type TPerson = object of RootObj name*: string # the * means that `name` is accessible from other modules age: int # no * means that the field is hidden from other modules TStudent = object of TPerson # TStudent inherits from TPerson id: int # with an id field var student: TStudent person: TPerson assert(student of TStudent) # is true # object construction: student = TStudent(name: "Anton", age: 5, id: 2) Object fields that should be visible from outside the defining module have to be marked by ``*``. In contrast to tuples, different object types are never *equivalent*. New object types can only be defined within a type section. Inheritance is done with the ``object of`` syntax. Multiple inheritance is currently not supported. If an object type has no suitable ancestor, ``RootObj`` can be used as its ancestor, but this is only a convention. Objects that have no ancestor are implicitly ``final``. You can use the ``inheritable`` pragma to introduce new object roots apart from ``system.RootObj``. (This is used in the GTK wrapper for instance.) **Note**: Composition (*has-a* relation) is often preferable to inheritance (*is-a* relation) for simple code reuse. Since objects are value types in Nim, composition is as efficient as inheritance. Mutually recursive types ------------------------ Objects, tuples and references can model quite complex data structures which depend on each other; they are *mutually recursive*. In Nim these types can only be declared within a single type section. (Anything else would require arbitrary symbol lookahead which slows down compilation.) Example: .. code-block:: nim type PNode = ref TNode # a traced reference to a TNode TNode = object le, ri: PNode # left and right subtrees sym: ref TSym # leaves contain a reference to a TSym TSym = object # a symbol name: string # the symbol's name line: int # the line the symbol was declared in code: PNode # the symbol's abstract syntax tree Type conversions ---------------- Nim distinguishes between `type casts`:idx: and `type conversions`:idx:. Casts are done with the ``cast`` operator and force the compiler to interpret a bit pattern to be of another type. Type conversions are a much more polite way to convert a type into another: They preserve the abstract *value*, not necessarily the *bit-pattern*. If a type conversion is not possible, the compiler complains or an exception is raised. The syntax for type conversions is ``destination_type(expression_to_convert)`` (like an ordinary call): .. code-block:: nim proc getID(x: TPerson): int = TStudent(x).id The ``InvalidObjectConversionError`` exception is raised if ``x`` is not a ``TStudent``. Object variants --------------- Often an object hierarchy is overkill in certain situations where simple variant types are needed. An example: .. code-block:: nim # This is an example how an abstract syntax tree could be modeled in Nim type TNodeKind = enum # the different node types nkInt, # a leaf with an integer value nkFloat, # a leaf with a float value nkString, # a leaf with a string value nkAdd, # an addition nkSub, # a subtraction nkIf # an if statement PNode = ref TNode TNode = object case kind: TNodeKind # the ``kind`` field is the discriminator of nkInt: intVal: int of nkFloat: floatVal: float of nkString: strVal: string of nkAdd, nkSub: leftOp, rightOp: PNode of nkIf: condition, thenPart, elsePart: PNode var n = PNode(kind: nkFloat, floatVal: 1.0) # the following statement raises an `FieldError` exception, because # n.kind's value does not fit: n.strVal = "" As can been seen from the example, an advantage to an object hierarchy is that no conversion between different object types is needed. Yet, access to invalid object fields raises an exception. Methods ------- In ordinary object oriented languages, procedures (also called *methods*) are bound to a class. This has disadvantages: * Adding a method to a class the programmer has no control over is impossible or needs ugly workarounds. * Often it is unclear where the method should belong to: is ``join`` a string method or an array method? Nim avoids these problems by not assigning methods to a class. All methods in Nim are multi-methods. As we will see later, multi-methods are distinguished from procs only for dynamic binding purposes. Method call syntax ------------------ There is a syntactic sugar for calling routines: The syntax ``obj.method(args)`` can be used instead of ``method(obj, args)``. If there are no remaining arguments, the parentheses can be omitted: ``obj.len`` (instead of ``len(obj)``). This method call syntax is not restricted to objects, it can be used for any type: .. code-block:: nim echo("abc".len) # is the same as echo(len("abc")) echo("abc".toUpper()) echo({'a', 'b', 'c'}.card) stdout.writeln("Hallo") # the same as writeln(stdout, "Hallo") (Another way to look at the method call syntax is that it provides the missing postfix notation.) So "pure object oriented" code is easy to write: .. code-block:: nim import strutils stdout.writeln("Give a list of numbers (separated by spaces): ") stdout.write(stdin.readLine.split.map(parseInt).max.`$`) stdout.writeln(" is the maximum!") Properties ---------- As the above example shows, Nim has no need for *get-properties*: Ordinary get-procedures that are called with the *method call syntax* achieve the same. But setting a value is different; for this a special setter syntax is needed: .. code-block:: nim type TSocket* = object of RootObj FHost: int # cannot be accessed from the outside of the module # the `F` prefix is a convention to avoid clashes since # the accessors are named `host` proc `host=`*(s: var TSocket, value: int) {.inline.} = ## setter of hostAddr s.FHost = value proc host*(s: TSocket): int {.inline.} = ## getter of hostAddr s.FHost var s: TSocket s.host = 34 # same as `host=`(s, 34) (The example also shows ``inline`` procedures.) The ``[]`` array access operator can be overloaded to provide `array properties`:idx:\ : .. code-block:: nim type TVector* = object x, y, z: float proc `[]=`* (v: var TVector, i: int, value: float) = # setter case i of 0: v.x = value of 1: v.y = value of 2: v.z = value else: assert(false) proc `[]`* (v: TVector, i: int): float = # getter case i of 0: result = v.x of 1: result = v.y of 2: result = v.z else: assert(false) The example is silly, since a vector is better modelled by a tuple which already provides ``v[]`` access. Dynamic dispatch ---------------- Procedures always use static dispatch. For dynamic dispatch replace the ``proc`` keyword by ``method``: .. code-block:: nim type PExpr = ref object of RootObj ## abstract base class for an expression PLiteral = ref object of PExpr x: int PPlusExpr = ref object of PExpr a, b: PExpr # watch out: 'eval' relies on dynamic binding method eval(e: PExpr): int = # override this base method quit "to override!" method eval(e: PLiteral): int = e.x method eval(e: PPlusExpr): int = eval(e.a) + eval(e.b) proc newLit(x: int): PLiteral = PLiteral(x: x) proc newPlus(a, b: PExpr): PPlusExpr = PPlusExpr(a: a, b: b) echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4))) Note that in the example the constructors ``newLit`` and ``newPlus`` are procs because it makes more sense for them to use static binding, but ``eval`` is a method because it requires dynamic binding. In a multi-method all parameters that have an object type are used for the dispatching: .. code-block:: nim type TThing = object of RootObj TUnit = object of TThing x: int method collide(a, b: TThing) {.inline.} = quit "to override!" method collide(a: TThing, b: TUnit) {.inline.} = echo "1" method collide(a: TUnit, b: TThing) {.inline.} = echo "2" var a, b: TUnit collide(a, b) # output: 2 As the example demonstrates, invocation of a multi-method cannot be ambiguous: Collide 2 is preferred over collide 1 because the resolution works from left to right. Thus ``TUnit, TThing`` is preferred over ``TThing, TUnit``. **Perfomance note**: Nim does not produce a virtual method table, but generates dispatch trees. This avoids the expensive indirect branch for method calls and enables inlining. However, other optimizations like compile time evaluation or dead code elimination do not work with methods. Exceptions ========== In Nim exceptions are objects. By convention, exception types are suffixed with 'Error'. The `system <system.html>`_ module defines an exception hierarchy that you might want to stick to. Exceptions derive from ``system.Exception``, which provides the common interface. Exceptions have to be allocated on the heap because their lifetime is unknown. The compiler will prevent you from raising an exception created on the stack. All raised exceptions should at least specify the reason for being raised in the ``msg`` field. A convention is that exceptions should be raised in *exceptional* cases: For example, if a file cannot be opened, this should not raise an exception since this is quite common (the file may not exist). Raise statement --------------- Raising an exception is done with the ``raise`` statement: .. code-block:: nim var e: ref OSError new(e) e.msg = "the request to the OS failed" raise e If the ``raise`` keyword is not followed by an expression, the last exception is *re-raised*. For the purpose of avoiding repeating this common code pattern, the template ``newException`` in the ``system`` module can be used: .. code-block:: nim raise newException(OSError, "the request to the OS failed") Try statement ------------- The ``try`` statement handles exceptions: .. code-block:: nim # read the first two lines of a text file that should contain numbers # and tries to add them var f: File if open(f, "numbers.txt"): try: let a = readLine(f) let b = readLine(f) echo "sum: ", parseInt(a) + parseInt(b) except OverflowError: echo "overflow!" except ValueError: echo "could not convert string to integer" except IOError: echo "IO error!" except: echo "Unknown exception!" # reraise the unknown exception: raise finally: close(f) The statements after the ``try`` are executed unless an exception is raised. Then the appropriate ``except`` part is executed. The empty ``except`` part is executed if there is an exception that is not explicitly listed. It is similar to an ``else`` part in ``if`` statements. If there is a ``finally`` part, it is always executed after the exception handlers. The exception is *consumed* in an ``except`` part. If an exception is not handled, it is propagated through the call stack. This means that often the rest of the procedure - that is not within a ``finally`` clause - is not executed (if an exception occurs). If you need to *access* the actual exception object or message inside an ``except`` branch you can use the `getCurrentException() <system.html#getCurrentException>`_ and `getCurrentExceptionMsg() <system.html#getCurrentExceptionMsg>`_ procs from the `system <system.html>`_ module. Example: .. code-block:: nim try: doSomethingHere() except: let e = getCurrentException() msg = getCurrentExceptionMsg() echo "Got exception ", repr(e), " with message ", msg Annotating procs with raised exceptions --------------------------------------- Through the use of the optional ``{.raises.}`` pragma you can specify that a proc is meant to raise a specific set of exceptions, or none at all. If the ``{.raises.}`` pragma is used, the compiler will verify that this is true. For instance, if you specify that a proc raises ``IOError``, and at some point it (or one of the procs it calls) starts raising a new exception the compiler will prevent that proc from compiling. Usage example: .. code-block:: nim proc complexProc() {.raises: [IOError, ArithmeticError].} = ... proc simpleProc() {.raises: [].} = ... Once you have code like this in place, if the list of raised exception changes the compiler will stop with an error specifying the line of the proc which stopped validating the pragma and the raised exception not being caught, along with the file and line where the uncaught exception is being raised, which may help you locate the offending code which has changed. If you want to add the ``{.raises.}`` pragma to existing code, the compiler can also help you. You can add the ``{.effects.}`` pragma statement to your proc and the compiler will output all inferred effects up to that point (exception tracking is part of Nim's effect system). Another more roundabout way to find out the list of exceptions raised by a proc is to use the Nim ``doc2`` command which generates documentation for a whole module and decorates all procs with the list of raised exceptions. You can read more about Nim's `effect system and related pragmas in the manual <manual.html#effect-system>`_. Generics ======== Generics are Nim's means to parametrize procs, iterators or types with `type parameters`:idx:. They are most useful for efficient type safe containers: .. code-block:: nim type TBinaryTree[T] = object # TBinaryTree is a generic type with # with generic param ``T`` le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil data: T # the data stored in a node PBinaryTree*[T] = ref TBinaryTree[T] # type that is exported proc newNode*[T](data: T): PBinaryTree[T] = # constructor for a node new(result) result.data = data proc add*[T](root: var PBinaryTree[T], n: PBinaryTree[T]) = # insert a node into the tree if root == nil: root = n else: var it = root while it != nil: # compare the data items; uses the generic ``cmp`` proc # that works for any type that has a ``==`` and ``<`` operator var c = cmp(it.data, n.data) if c < 0: if it.le == nil: it.le = n return it = it.le else: if it.ri == nil: it.ri = n return it = it.ri proc add*[T](root: var PBinaryTree[T], data: T) = # convenience proc: add(root, newNode(data)) iterator preorder*[T](root: PBinaryTree[T]): T = # Preorder traversal of a binary tree. # Since recursive iterators are not yet implemented, # this uses an explicit stack (which is more efficient anyway): var stack: seq[PBinaryTree[T]] = @[root] while stack.len > 0: var n = stack.pop() while n != nil: yield n.data add(stack, n.ri) # push right subtree onto the stack n = n.le # and follow the left pointer var root: PBinaryTree[string] # instantiate a PBinaryTree with ``string`` add(root, newNode("hello")) # instantiates ``newNode`` and ``add`` add(root, "world") # instantiates the second ``add`` proc for str in preorder(root): stdout.writeln(str) The example shows a generic binary tree. Depending on context, the brackets are used either to introduce type parameters or to instantiate a generic proc, iterator or type. As the example shows, generics work with overloading: the best match of ``add`` is used. The built-in ``add`` procedure for sequences is not hidden and is used in the ``preorder`` iterator. Templates ========= Templates are a simple substitution mechanism that operates on Nim's abstract syntax trees. Templates are processed in the semantic pass of the compiler. They integrate well with the rest of the language and share none of C's preprocessor macros flaws. To *invoke* a template, call it like a procedure. Example: .. code-block:: nim template `!=` (a, b: expr): expr = # this definition exists in the System module not (a == b) assert(5 != 6) # the compiler rewrites that to: assert(not (5 == 6)) The ``!=``, ``>``, ``>=``, ``in``, ``notin``, ``isnot`` operators are in fact templates: this has the benefit that if you overload the ``==`` operator, the ``!=`` operator is available automatically and does the right thing. (Except for IEEE floating point numbers - NaN breaks basic boolean logic.) ``a > b`` is transformed into ``b < a``. ``a in b`` is transformed into ``contains(b, a)``. ``notin`` and ``isnot`` have the obvious meanings. Templates are especially useful for lazy evaluation purposes. Consider a simple proc for logging: .. code-block:: nim const debug = true proc log(msg: string) {.inline.} = if debug: stdout.writeln(msg) var x = 4 log("x has the value: " & $x) This code has a shortcoming: if ``debug`` is set to false someday, the quite expensive ``$`` and ``&`` operations are still performed! (The argument evaluation for procedures is *eager*). Turning the ``log`` proc into a template solves this problem: .. code-block:: nim const debug = true template log(msg: string) = if debug: stdout.writeln(msg) var x = 4 log("x has the value: " & $x) The parameters' types can be ordinary types or the meta types ``expr`` (stands for *expression*), ``stmt`` (stands for *statement*) or ``typedesc`` (stands for *type description*). If the template has no explicit return type, ``stmt`` is used for consistency with procs and methods. If there is a ``stmt`` parameter it should be the last in the template declaration. The reason is that statements can be passed to a template via a special ``:`` syntax: .. code-block:: nim template withFile(f: expr, filename: string, mode: FileMode, body: stmt): stmt {.immediate.} = let fn = filename var f: File if open(f, fn, mode): try: body finally: close(f) else: quit("cannot open: " & fn) withFile(txt, "ttempl3.txt", fmWrite): txt.writeln("line 1") txt.writeln("line 2") In the example the two ``writeln`` statements are bound to the ``body`` parameter. The ``withFile`` template contains boilerplate code and helps to avoid a common bug: to forget to close the file. Note how the ``let fn = filename`` statement ensures that ``filename`` is evaluated only once. Macros ====== Macros enable advanced compile-time code transformations, but they cannot change Nim's syntax. However, this is no real restriction because Nim's syntax is flexible enough anyway. Macros have to be implemented in pure Nim code if the `foreign function interface (FFI) <manual.html#foreign-function-interface>`_ is not enabled in the compiler, but other than that restriction (which at some point in the future will go away) you can write any kind of Nim code and the compiler will run it at compile time. There are two ways to write a macro, either *generating* Nim source code and letting the compiler parse it, or creating manually an abstract syntax tree (AST) which you feed to the compiler. In order to build the AST one needs to know how the Nim concrete syntax is converted to an abstract syntax tree (AST). The AST is documented in the `macros <macros.html>`_ module. Once your macro is finished, there are two ways to invoke it: (1) invoking a macro like a procedure call (expression macros) (2) invoking a macro with the special ``macrostmt`` syntax (statement macros) Expression Macros ----------------- The following example implements a powerful ``debug`` command that accepts a variable number of arguments: .. code-block:: nim # to work with Nim syntax trees, we need an API that is defined in the # ``macros`` module: import macros macro debug(n: varargs[expr]): stmt = # `n` is a Nim AST that contains a list of expressions; # this macro returns a list of statements: result = newNimNode(nnkStmtList, n) # iterate over any argument that is passed to this macro: for i in 0..n.len-1: # add a call to the statement list that writes the expression; # `toStrLit` converts an AST to its string representation: result.add(newCall("write", newIdentNode("stdout"), toStrLit(n[i]))) # add a call to the statement list that writes ": " result.add(newCall("write", newIdentNode("stdout"), newStrLitNode(": "))) # add a call to the statement list that writes the expressions value: result.add(newCall("writeln", newIdentNode("stdout"), n[i])) var a: array[0..10, int] x = "some string" a[0] = 42 a[1] = 45 debug(a[0], a[1], x) The macro call expands to: .. code-block:: nim write(stdout, "a[0]") write(stdout, ": ") writeln(stdout, a[0]) write(stdout, "a[1]") write(stdout, ": ") writeln(stdout, a[1]) write(stdout, "x") write(stdout, ": ") writeln(stdout, x) Statement Macros ---------------- Statement macros are defined just as expression macros. However, they are invoked by an expression following a colon. The following example outlines a macro that generates a lexical analyzer from regular expressions: .. code-block:: nim macro case_token(n: stmt): stmt = # creates a lexical analyzer from regular expressions # ... (implementation is an exercise for the reader :-) discard case_token: # this colon tells the parser it is a macro statement of r"[A-Za-z_]+[A-Za-z_0-9]*": return tkIdentifier of r"0-9+": return tkInteger of r"[\+\-\*\?]+": return tkOperator else: return tkUnknown Term rewriting macros --------------------- Term rewriting macros can be used to enhance the compilation process with user defined optimizations; see this `document <trmacros.html>`_ for further information. Building your first macro ------------------------- To give a footstart to writing macros we will show now how to turn your typical dynamic code into something that compiles statically. For the exercise we will use the following snippet of code as the starting point: .. code-block:: nim import strutils, tables proc readCfgAtRuntime(cfgFilename: string): Table[string, string] = let inputString = readFile(cfgFilename) var source = "" result = initTable[string, string]() for line in inputString.splitLines: # Ignore empty lines if line.len < 1: continue var chunks = split(line, ',') if chunks.len != 2: quit("Input needs comma split values, got: " & line) result[chunks[0]] = chunks[1] if result.len < 1: quit("Input file empty!") let info = readCfgAtRuntime("data.cfg") when isMainModule: echo info["licenseOwner"] echo info["licenseKey"] echo info["version"] Presumably this snippet of code could be used in a commercial software, reading a configuration file to display information about the person who bought the software. This external file would be generated by an online web shopping cart to be included along the program containing the license information:: version,1.1 licenseOwner,Hyori Lee licenseKey,M1Tl3PjBWO2CC48m The ``readCfgAtRuntime`` proc will open the given filename and return a ``Table`` from the `tables module <tables.html>`_. The parsing of the file is done (without much care for handling invalid data or corner cases) using the `splitLines proc from the strutils module <strutils.html#splitLines>`_. There are many things which can fail; mind the purpose is explaining how to make this run at compile time, not how to properly implement a DRM scheme. The reimplementation of this code as a compile time proc will allow us to get rid of the ``data.cfg`` file we would need to distribute along the binary, plus if the information is really constant, it doesn't make from a logical point of view to have it *mutable* in a global variable, it would be better if it was a constant. Finally, and likely the most valuable feature, we can implement some verification at compile time. You could think of this as a *better unit testing*, since it is impossible to obtain a binary unless everything is correct, preventing you to ship to users a broken program which won't start because a small critical file is missing or its contents changed by mistake to something invalid. Generating source code ++++++++++++++++++++++ Our first attempt will start by modifying the program to generate a compile time string with the *generated source code*, which we then pass to the ``parseStmt`` proc from the `macros module <macros.html>`_. Here is the modified source code implementing the macro: .. code-block:: nim :number-lines: import macros, strutils macro readCfgAndBuildSource(cfgFilename: string): stmt = let inputString = slurp(cfgFilename.strVal) var source = "" for line in inputString.splitLines: # Ignore empty lines if line.len < 1: continue var chunks = split(line, ',') if chunks.len != 2: error("Input needs comma split values, got: " & line) source &= "const cfg" & chunks[0] & "= \"" & chunks[1] & "\"\n" if source.len < 1: error("Input file empty!") result = parseStmt(source) readCfgAndBuildSource("data.cfg") when isMainModule: echo cfglicenseOwner echo cfglicenseKey echo cfgversion The good news is not much has changed! First, we need to change the handling of the input parameter (line 3). In the dynamic version the ``readCfgAtRuntime`` proc receives a string parameter. However, in the macro version it is also declared as string, but this is the *outside* interface of the macro. When the macro is run, it actually gets a ``PNimNode`` object instead of a string, and we have to call the `strVal proc <macros.html#strVal>`_ (line 5) from the `macros module <macros.html>`_ to obtain the string being passed in to the macro. Second, we cannot use the `readFile proc <system.html#readFile>`_ from the `system module <system.html>`_ due to FFI restriction at compile time. If we try to use this proc, or any other which depends on FFI, the compiler will error with the message ``cannot evaluate`` and a dump of the macro's source code, along with a stack trace where the compiler reached before bailing out. We can get around this limitation by using the `slurp proc <system.html#slurp>`_ from the `system module <system.html>`_, which was precisely made for compilation time (just like `gorge <system.html#gorge>`_ which executes an external program and captures its output). The interesting thing is that our macro does not return a runtime `Table <tables.html#TTable>`_ object. Instead, it builds up Nim source code into the ``source`` variable. For each line of the configuration file a ``const`` variable will be generated (line 15). To avoid conflicts we prefix these variables with ``cfg``. In essence, what the compiler is doing is replacing the line calling the macro with the following snippet of code: .. code-block:: nim const cfgversion= "1.1" const cfglicenseOwner= "Hyori Lee" const cfglicenseKey= "M1Tl3PjBWO2CC48m" You can verify this yourself adding the line ``echo source`` somewhere at the end of the macro and compiling the program. Another difference is that instead of calling the usual `quit proc <system.html#quit>`_ to abort (which we could still call) this version calls the `error proc <macros.html#error>`_ (line 14). The ``error`` proc has the same behavior as ``quit`` but will dump also the source and file line information where the error happened, making it easier for the programmer to find where compilation failed. In this situation it would point to the line invoking the macro, but **not** the line of ``data.cfg`` we are processing, that's something the macro itself would need to control. Generating AST by hand ++++++++++++++++++++++ To generate an AST we would need to intimately know the structures used by the Nim compiler exposed in the `macros module <macros.html>`_, which at first look seems a daunting task. But we can use as helper shortcut the `dumpTree macro <macros.html#dumpTree>`_, which is used as a statement macro instead of an expression macro. Since we know that we want to generate a bunch of ``const`` symbols we can create the following source file and compile it to see what the compiler *expects* from us: .. code-block:: nim import macros dumpTree: const cfgversion: string = "1.1" const cfglicenseOwner= "Hyori Lee" const cfglicenseKey= "M1Tl3PjBWO2CC48m" During compilation of the source code we should see the following lines in the output (again, since this is a macro, compilation is enough, you don't have to run any binary):: StmtList ConstSection ConstDef Ident !"cfgversion" Ident !"string" StrLit 1.1 ConstSection ConstDef Ident !"cfglicenseOwner" Empty StrLit Hyori Lee ConstSection ConstDef Ident !"cfglicenseKey" Empty StrLit M1Tl3PjBWO2CC48m With this output we have a better idea of what kind of input the compiler expects. We need to generate a list of statements. For each constant the source code generates a ``ConstSection`` and a ``ConstDef``. If we were to move all the constants to a single ``const`` block we would see only a single ``ConstSection`` with three children. Maybe you didn't notice, but in the ``dumpTree`` example the first constant explicitly specifies the type of the constant. That's why in the tree output the two last constants have their second child ``Empty`` but the first has a string identifier. So basically a ``const`` definition is made up from an identifier, optionally a type (can be an *empty* node) and the value. Armed with this knowledge, let's look at the finished version of the AST building macro: .. code-block:: nim :number-lines: import macros, strutils macro readCfgAndBuildAST(cfgFilename: string): stmt = let inputString = slurp(cfgFilename.strVal) result = newNimNode(nnkStmtList) for line in inputString.splitLines: # Ignore empty lines if line.len < 1: continue var chunks = split(line, ',') if chunks.len != 2: error("Input needs comma split values, got: " & line) var section = newNimNode(nnkConstSection) constDef = newNimNode(nnkConstDef) constDef.add(newIdentNode("cfg" & chunks[0])) constDef.add(newEmptyNode()) constDef.add(newStrLitNode(chunks[1])) section.add(constDef) result.add(section) if result.len < 1: error("Input file empty!") readCfgAndBuildAST("data.cfg") when isMainModule: echo cfglicenseOwner echo cfglicenseKey echo cfgversion Since we are building on the previous example generating source code, we will only mention the differences to it. Instead of creating a temporary ``string`` variable and writing into it source code as if it were written *by hand*, we use the ``result`` variable directly and create a statement list node (``nnkStmtList``) which will hold our children (line 7). For each input line we have to create a constant definition (``nnkConstDef``) and wrap it inside a constant section (``nnkConstSection``). Once these variables are created, we fill them hierarchichally (line 17) like the previous AST dump tree showed: the constant definition is a child of the section definition, and the constant definition has an identifier node, an empty node (we let the compiler figure out the type), and a string literal with the value. A last tip when writing a macro: if you are not sure the AST you are building looks ok, you may be tempted to use the ``dumpTree`` macro. But you can't use it *inside* the macro you are writting/debugging. Instead ``echo`` the string generated by `treeRepr <macros.html#treeRepr>`_. If at the end of the this example you add ``echo treeRepr(result)`` you should get the same output as using the ``dumpTree`` macro, but of course you can call that at any point of the macro where you might be having troubles.


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