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 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 or user guide 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, preferring 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:

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.)

Ref objects should be used whenever inheritance is used. It isn’t strictly necessary, but with non-ref objects assignments such as let person: Person = Student(id: 123) will truncate subclass fields.

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:

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):

The InvalidObjectConversionError exception is raised if x is not a Student.

Object variants ¶

Often an object hierarchy is overkill in certain situations where simple variant types are needed.

An example:

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:

(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:

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:

(The example also shows inline procedures.)

The [] array access operator can be overloaded to provide `array properties`:idx::

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:

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:

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 Unit, Thing is preferred over Thing, Unit.

Performance 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 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:

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:

Try statement ¶

The try statement handles exceptions:

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() and getCurrentExceptionMsg() procs from the system module. Example:

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:

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.

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:

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:

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:

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:

The parameters’ types can be ordinary types or the meta types untyped, typed, or typedesc. typedesc stands for type description, and untyped means symbol lookups and type resolution is not performed before the expression is passed to the template.

If the template has no explicit return type, void is used for consistency with procs and methods.

To pass a block of statements to a template, use ‘untyped’ for the last parameter:

In the example the two writeLine 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) 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 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:

The macro call expands to:

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:

macro case_token(n: varargs[untyped]): typed =
  # 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

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:

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. 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. 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. Here is the modified source code implementing the macro:

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 (line 5) from the macros module to obtain the string being passed in to the macro.

Second, we cannot use the readFile proc from the system module 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 from the system module, which was precisely made for compilation time (just like gorge which executes an external program and captures its output).

The interesting thing is that our macro does not return a runtime Table 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:

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 to abort (which we could still call) this version calls the error proc (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, which at first look seems a daunting task. But we can use as helper shortcut the dumpTree macro, 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:

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:

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. 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.

Example Templates and Macros ¶

Compilation to JavaScript ¶

Nim code can be compiled to JavaScript. However in order to write JavaScript-compatible code you should remember the following: - addr and ptr have slightly different semantic meaning in JavaScript.

It is recommended to avoid those if you’re not sure how they are translated to JavaScript.

cast[T](x) in JavaScript is translated to (x), except for casting between signed/unsigned ints, in which case it behaves as static cast in C language.
cstring in JavaScript means JavaScript string. It is a good practice to use cstring only when it is semantically appropriate. E.g. don’t use cstring as a binary data buffer.