Contra

• Lightweight and fast Self-Documenting Design by Contract Programming, Design by Contract, zero cost at runtime.
• Security Hardened mode (based from Debian Hardened & Gentoo Hardened, checked with hardening-check).
• Change Immutable variables, into Immutable variables, Immutable programming.

• Produces no code at all when build for release, KISS
• Works on NimScript, JavaScript, {.compiletime.} and static:.

Use

Design By Contract

import contra

func funcWithContract(mustBePositive: int): int =
  preconditions mustBePositive > 0, mustBePositive > -1 ## Require (Preconditions)
  postconditions result > 0, result < int32.high        ## Ensure  (Postconditions)

  result = mustBePositive - 1 ## Mimic some logic, notice theres no "body" block

discard funcWithContract(2)
# discard funcWithContract(0)  # Uncomment to see it fail as expected.

Hardened mode

import contra
hardenedBuild()  # Security Hardened mode enabled, compile with:  -d:hardened
echo "Hello World"

Changing Immutable Variables

import contra

type Person = object # Changing Immutable Variables,into Immutable Variables.
  name: string
  age: Natural

let
  bob = Person(name: "Bob", age: 42)  # Immutable Variable, original.
  olderBob = bob.deepCopy:            # Immutable Variable, but changed.
    this.age = 45
    this.name = this.name[0..^2]

echo bob       # (name: "Bob", age: 42)      Original Immutable
echo olderBob  # (name: "Bo", age: 45)       Changed Immutable

Its inspired by Scala:

val immutableButChanged = immutable.copy(attribute = 9)

Assercho

assert(conditionBool, errorString) + echo(Nim_Code) + printf(C_Code) Combined "3-in-1".

• Is different than echo that you dont have to manually remove it from the code for Release builds.
• Is different than assert that it prints to terminal when conditionBool is true.

It only works when not defined(release) and not defined(danger) for Debugging purposes.

C Source code debug is similar to JS Source Maps, shows C code corresponding to the same Nim code.

The assertion is a vanilla assert, when fails produces an AssertionError.

Produces No code at all when build for Release, zero runtime performance cost. assercho is a Macro.

assercho when assert is true (Ok)

import contra

let foo = 42
let bar = 9
assercho(foo > bar, "Assercho for all the Brochachos!")  # 42 > 9

Nim   foo > bar = true --> /home/juan/code/example.nim(3, 8)
C     (bar_1kqJRPNteJWdjlPR09aibuA < foo_FvFJfeatIb8qi8DN7lzoAQ) = 1 --> /tmp/example.nim.c(9)

assercho when assert is false (Error)

import contra

let foo = 42
let bar = 9
assercho(foo < bar, "Assercho for all the Brochachos!")  # 42 < 9

Nim   foo < bar = false --> /home/juan/code/example.nim(3, 8)
C     (bar_1kqJRPNteJWdjlPR09aibuA < foo_FvFJfeatIb8qi8DN7lzoAQ) = 0 --> /tmp/example.nim.c(9)
Error: unhandled exception: `foo < bar` Assercho for all the Brochachos! [AssertionError]

Replaces approx the following code:

when not defined(danger) and not defined(release):
  debugEcho "foo = ", foo
  debugEcho "bar = ", bar
  debugEcho foo > bar
  assert foo > bar, "Error message"
  # Custom Macro to get the Nim Source Code line info of origin (macros.NimNode.LineInfo)
  # Custom Macro/Template to get the C Source Code line info of origin

Compile-Time Term-Rewriting Template Optimizations

• echo() and debugEcho() gets Rewritten to use fwrite() from stdio.h directly. Logging is not touched.
• fwrite() is faster than puts(), puts() is faster than printf(). fwrite() > puts() > printf().
• Optimizations are optional, only get enabled when defined(release) and defined(danger).

echo "a", "b", "c"

Gets optimized to:

fwrite("abc\012", ((unsigned int) 1), ((unsigned int) 4), stdout);

• Float Division is slower than multiplication. 1 CPU Cycle for Multiplication, 100 CPU Cycles or more for Division.
• Float Division gets Rewritten to multiplication with the inverse.
• Example x / 3.0 --> x * static(1.0 / 3.0).
• Optimizations are optional, only get enabled when defined(release) and defined(danger).

var x, y = 2.0
echo x / 2.0

Gets optimized to:

NF x_9b3J8iZeIHRoRKYxMY9a9bzzQ;
x_9b3J8iZeIHRoRKYxMY9a9bzzQ = 2.0000000000000000e+00;
( (NF)(x_9b3J8iZeIHRoRKYxMY9a9bzzQ) * (NF)(5.0000000000000000e-01) );

• For more info check this commit.

Description

Contract Preconditions:

• preconditions takes preconditions separated by commas, asserts on arguments or local variables.

Contract Postconditions:

• postconditions takes postconditions separated by commas, must assert on result, can assert on local variables.

Contracts Preconditions and Postconditions:

• postconditions must be AFTER preconditions.
• postconditions must NOT be repeated.
• -d:contracts Force enable Contracts, can be used independently of -d:release.

Security Hardened Mode:

• -d:hardened Force enable Security Hardened mode, can be used independently of -d:release.
• -d:hardened requires -d:contracts.
• Security Hardened mode only works for default target backend.
• Produces no code at all if -d:hardened is not defined.
• hardenedBuild() is 1 Template, takes no arguments, returns nothing.
• hardenedBuild() must be called on the root top of your main module.
• Hardened build is ideal companion for a Contracts module, still optional anyway.

Changing Immutable Variables:

• deepCopy Lets you change Immutable Variables, into Immutable Variables, using Mutated copies. It mimic Scala's val immutableButChanged = immutable.copy(attribute = 9). Immutable programming.

Assercho:

• assert + echo = assercho. Assertive programming util.

Install

• nimble install contra

FAQ

• Why not just use Contracts ?

$ cat example.nim
import contracts
from math import sqrt, floor
proc isqrt[T: SomeInteger](x: T): T {.contractual.} =
  require:
    x >= 0
  ensure:
    result * result <= x
    (result+1) * (result+1) > x
  body:
    (T)(x.toBiggestFloat().sqrt().floor().toBiggestInt())
echo isqrt(18)
echo isqrt(-8)

$ nim js -r example.nim
Error: undeclared identifier: 'deepCopy'

$ nim e example.nim
Error: undeclared identifier: 'deepCopy'

$ cat example2compiletime.nim
import contracts
from math import sqrt, floor
proc isqrt[T: SomeInteger](x: T): T {.contractual, compiletime.} =
  require:
    x >= 0
  ensure:
    result * result <= x
    (result+1) * (result+1) > x
  body:
    (T)(x.toBiggestFloat().sqrt().floor().toBiggestInt())
echo isqrt(18)
echo isqrt(-8)

$ nim c -r example2compiletime.nim
Error: request to generate code for .compileTime proc: isqrt

$ cloc ~/.nimble/pkgs/contracts-0.1.0/
Language          files         blank        comment        code
----------------------------------------------------------------
Nim               21            119          515            640

• Whats Contract Programming, Design by Contract?.

https://www.youtube.com/watch?v=DRVoh5XiAZo

https://en.wikipedia.org/wiki/Defensive_programming#Other_techniques

http://stackoverflow.com/questions/787643/benefits-of-assertive-programming

https://en.wikipedia.org/wiki/Hoare_logic#Hoare_triple

• What about No Side Effects?.

https://nim-lang.org/docs/manual.html#procedures-func

https://nim-lang.org/docs/manual.html#pragmas-nosideeffect-pragma

• What about Types?.

https://nim-lang.org/docs/manual_experimental.html#concepts

• How to use this at Compile Time?.

Add {.compiletime.} or static:.

• What about assume blocks?.

Assume blocks produce no code at all and are only meant for human reading only, you can do that using discard or similar contruct on Nim. KISS.

• What about body blocks?.

This library does NOT uses nor needs body blocks.

• What about invariant blocks?.

You can pass Invariants on the postconditions block.

• What about forall and forsome blocks?.

Use sequtils.filterIt, sequtils.mapIt, sequtils.keepItIf, sequtils.allIt, sequtils.anyIt, etc.

• What about ghost block?.

Use when defined(release): or when defined(contracts):

• Whats the performance and speed cost of using Contra?.

Zero cost at runtime, since it produces no code at all when build for Release.

• I prefer the naming require and ensure ?.

from contra import preconditions as require
from contra import postconditions as ensure

• I prefer the naming pre and post ?.

from contra import preconditions as pre
from contra import postconditions as post

• If I add this to my project I am forced to use it everywhere?.

No.

The code will just work on blocks without Contract. You only need to add 2 lines to your existing code (1 for Preconditions, 1 for Postconditions). Is recommended to at least use it con "core" functionality.

• Whats Hardened mode ?.

https://en.wikipedia.org/wiki/Hardening_%28computing%29#Binary_hardening

• More Documentation?.

nim doc contra.nim

" TDD is Poor-Man's Contracts "