Contra

• Lightweight and fast Contract Programming, Design by Contract, on 9 lines of code.

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

Use

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.

• preconditions takes preconditions separated by commas, asserts on arguments or variables.
• postconditions takes postconditions separated by commas, must assert on result.
• -d:contracts Force enable Contracts, can be used independently of -d:release.

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

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

• 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.

• I prefer the naming require and ensure ?.

from contra import preconditions as require or similar.

• 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.

• More Documentation?.

nim doc contra.nim