Nim Imports

What I do

I provide guidelines for importing modules and common pitfalls in the Tattletale Nim project.

Project Directory Structure

/home/beta/Programming/Perso/tattletale/
├── config.nims                    # Project-level config with paths and tasks
├── workspace/
    └── mylib/                    # First-party libraries
        ├── mylib.nim            # Re-exports all public symbols
        ├── src/
        │   └── mylib.nim       # Implementation
        └── tests/
            └── test_mylib.nim  # Tests

Import Patterns

First-party workspace libraries

import workspace/mylib          # Imports from mylib.nim which re-exports

Note: Do NOT use import workspace/mylib/mylib - the .nim extension and file path are handled by the config.nims --path:"." setting.

Third-party packages (via nimble)

import pkg/jsony               # For packages installed via nimble

Standard library modules

import std/tables              # For stdlib modules
import std/os
import std/json
import std/sequtils           # IMPORTANT: Required for mapIt

Common Pitfalls

Missing std/sequtils for mapIt

Always import std/sequtils when using mapIt:

import std/sequtils  # MISSING import causes: "undeclared identifier: mapIt"
let nums = @[1, 2, 3]
let doubled = nums.mapIt(it * 2)  # Fails without sequtils import

Wrong import for workspace libraries

Wrong form (rejects):

import toktoktok               # Fails - not in stdlib or pkg
import workspace/toktoktok/toktoktok.nim  # Wrong path

Correct form (compiles):

import workspace/toktoktok    # Imports the re-export module

Confusing pkg/ vs workspace/

• pkg/ - Third-party packages installed via nimble (e.g., jsony, chronos)
• workspace/ - First-party project libraries

import pkg/jsony              # Third-party JSON library
import workspace/toktoktok   # Your project's tokenizer

Accessing Private Procedures

To access private (non-*) procedures from a module, use the {.all.} pragma:

import workspace/toktoktok {.all.}

# Now you can access bytePairEncode and bytePairMerge even though they were private
let ranks = initTable[seq[byte], int]()
ranks[@[byte(228), byte(189)]] = 19526
let result = bytePairEncode(piece, ranks)

Accessing Private Fields

For accessing private fields of objects, use std/importutils with privateAccess:

# file_a.nim
type MyObjectA* = object # public
  privateFieldA: int     # privaterivate

# file_b.nim
type MyObjectB = object  # private
  privateFieldB: int     # private

# file_c.nim
import std/importutils
import file_a            # MyObjectA is public
import file_b {.all.}    # MyObjectB is private

block:
    privateAccess(MyObjectA)
    let obj = MyObject(privateFieldA: 42)
    echo obj.privateFieldA  # Accessible in this scope
block:
    privateAccess(MyObjectB)
    let obj = MyObject(privateFieldB: 42)
    echo obj.privateFieldB  # Accessible in this scope

Re-export Pattern

Each workspace library should have a re-export module at the root:

# workspace/mylib/mylib.nim
# Re-exports all public symbols from src/ and submodules

export src/mylib

This allows users to import workspace/mylib instead of knowing the internal structure.

PCRE2 Notes

PCRE2 is vendored in workspace/pcre2. The config.nims automatically sets:

• Include paths for headers
• Platform-specific defines
• PCRE2-specific configuration

No manual configuration needed - just import workspace/pcre2.

Config.nims Setup

Project root config.nims sets:

• --path:"." - Allows import workspace/foo syntax
• PCRE2 compile flags
• Test task commands

Test discovery rules:

• Files in workspace/*/tests/ directory
• Filenames with the test_ prefix or the t_ prefix