Macros in Nim are powerful tools that allow you to transform Nim’s abstract syntax tree (AST) at compile-time. This tutorial will guide you through the essential concepts and practical applications of Nim’s macro system.

What is a Macro?

A macro in Nim is a function that executes at compile-time, transforming a Nim syntax tree into a different tree. This capability can be leveraged for various tasks, such as:

  1. Enhanced Assertions: Implement an assert macro that prints both sides of a comparison if the assertion fails.
  2. Debugging: Create a debug macro that prints the value and name of a symbol.
  3. Symbolic Differentiation: Automatically perform differentiation on mathematical expressions.

Example Implementations

  1. Assertion Macro:

      macro myAssert(arg: untyped): untyped =
  arg.expectKind nnkInfix
  arg.expectLen 3
  let op = newLit(" " & arg[0].repr & " ")
  let lhs = arg[1]
  let rhs = arg[2]

  result = quote do:
    if not `arg`:
      raise newException(AssertionDefect, $`lhs` & `op` & $`rhs`)

let a = 1
let b = 2
myAssert(a != b)
myAssert(a == b)

  2. Debug Macro:

      macro myDebugEcho(arg: untyped): untyped =
  result = quote do:
    echo `arg.repr`, ": ", `arg`

let x = 42
myDebugEcho(x)

  3. Symbolic Differentiation:

      import macros

macro diff(expr: untyped, var: untyped): untyped =
  # Implementation of symbolic differentiation goes here
  # This example is a placeholder
  result = quote do:
    echo "Differentiation logic"

let a = 2
let b = 3
diff(a * b, a)

Macro Arguments

Macro arguments can be typed or untyped, each with its advantages and limitations.

  • Untyped Arguments: Passed before semantic checks, leading to simpler syntax trees but less flexibility with Nim’s overload resolution.
  • Typed Arguments: Passed after semantic checks, offering more complex trees with type information.

Static Arguments

Static arguments pass values directly to the macro, not as syntax trees.

  import std/macros

macro myMacro(arg: static[int]): untyped =
  echo arg # Outputs the integer value directly

myMacro(1 + 2 * 3) # Outputs: 7

Code Blocks as Arguments

You can pass complex syntax trees to macros using indented code blocks.

  echo "Hello ":
  let a = "Wor"
  let b = "ld!"
  a & b

This feature is particularly useful for macros, allowing the passage of complex trees.

Understanding the Syntax Tree

To build and manipulate Nim’s syntax tree, it’s crucial to understand how Nim source code is represented. The macros.treeRepr function is invaluable for this, as it converts a syntax tree into a readable string.

  import macros

dumpTree:
  var mt: MyType = MyType(a:123.456, b:"abcdef")

# Output example:
# StmtList
#   VarSection
#     IdentDefs
#       Ident "mt"
#       Ident "MyType"
#       ObjConstr
#         Ident "MyType"
#         ExprColonExpr
#           Ident "a"
#           FloatLit 123.456
#         ExprColonExpr
#           Ident "b"
#           StrLit "abcdef"

Custom Semantic Checking

Before transforming the syntax tree, macros should verify the form of their arguments.

  macro myAssert(arg: untyped): untyped =
  arg.expectKind nnkInfix
  arg.expectLen 3
  # Further processing...

Generating Code

You can generate code in two ways:

  1. Using newTree and newLit:

      import std/macros

let tree = newTree(nnkStmtList, newLit(42))

  2. Using quote do:

      import std/macros

macro a(i) = quote do:
  let `i` = 0

a b
doAssert b == 0

Building Your First Macro

To create a macro like myAssert, start with a simple example and print the argument to understand its structure.

  import std/macros

macro myAssert(arg: untyped): untyped =
  echo arg.treeRepr

let a = 1
let b = 2

myAssert(a != b)

Power and Responsibility

Macros are powerful and should be used judiciously. Prefer templates or generics when possible, and ensure macros are well-documented to maintain code clarity.

Limitations

Macros share the limitations of the NimVM, such as the inability to call non-compiler C functions directly.

By understanding these core concepts and techniques, you can harness the full power of Nim’s macro system to write more expressive and efficient code.

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Last updated 17 Aug 2024, 12:31 +0200 . history