This Month with Nim: June 2021


Authors: @Leorize, @disruptek, @Zevv and @saem

The project brings Continuation-Passing Style to Nim. CPS is an elegant approach to writing control-flow which is rooted in musty programming language theory. Now Nim’s macro system allows you the advantages of CPS without having to introduce any syntax.

A short demonstration of implementing goto in Nim with CPS:

import std/tables
import cps

  Count = ref object of Continuation
    labels: Table[string, Continuation.fn]

proc label(c: Count; name: string): Count {.cpsMagic.} =
  c.labels[name] = c.fn
  result = c

proc goto(c: Count; name: string): Count {.cpsMagic.} =
  c.fn = c.labels[name]
  result = c

proc count(upto: int): int {.cps: Count.} =
  ## deploy the Count to make counting fun again;
  ## this continuation returns the number of trips through the goto
  result = 0
  label: "again!"
  inc result
  echo result, "!"
  echo result, " loops, ah ah ah!"
  if result < upto:
    goto "again!"
  echo "whew!"

const many = 1_000
assert many == count(many) # This might take awhile to run

If you are interested, see Zevv’s excellent write up on CPS. Come join us at or #cps on


Author: @Thisago

The Whois.nim is a simple whois client. With cache!

import whois

echo whois("")

# or

var domain = "".toDomain # convert to a `Domain` instance
domain.update() # Get data from API

echo domain


Author: @planetis-m

Thin interface for LLVM/Clang libFuzzer, an in-process, coverage-guided, evolutionary fuzzing engine. Fuzzing is an automated bug finding technique, where randomized inputs are fed to a target program in order to get it to crash. With fuzzing, you can increase your test coverage to find edge cases and trigger bugs more effectively.

proc fuzzMe(data: openarray[byte]): bool =
  result = data.len >= 3 and
    data[0].char == 'F' and
    data[1].char == 'U' and
    data[2].char == 'Z' and
    data[3].char == 'Z' # :‑<

proc initialize(): cint {.exportc: "LLVMFuzzerInitialize".} =
  {.emit: "N_CDECL(void, NimMain)(void); NimMain();".}

proc testOneInput(data: ptr UncheckedArray[byte], len: int): cint {.
    exportc: "LLVMFuzzerTestOneInput", raises: [].} =
  result = 0
  discard fuzzMe(data.toOpenArray(0, len-1))

It takes a split second for libFuzzer to perform ~40.000 runs. Behind the scenes it uses value profiling to guide the fuzzer past these comparisons much more efficiently than simply hoping to stumble on the exact sequence of bytes by chance.


Author: Juan Carlos

Dik is a dictionary implemented as { array[char]: Option[T] }

  • Cute pet.
  • Destructors.
  • Resize in-place.
  • Sorted and hashed.
  • Same size as Table.
  • Dollar and pretty.
  • toSeq and toDik.
  • Tests for everything.
  • Documentation with examples.
  • Same size and speed as Table.
  • 1 file, 300 lines, 0 dependencies.
  • newDikOfCap for custom capacity.
  • Keys are stored as array of char.
  • Values are stored as seq of Option[T].
  • Can be used like seq or like Table.
  • len for lenght and cap for capacity.
  • Get items by string or openArray[char].
  • Get items by index or BackwardsIndex or Slice[int].
  • Iterators, including enumerated yields (index, key, value).
  • From stdlib uses only options for Option[T], hashes for hash(T).
import std/options ## For Option[T] ops.
import std/json    ## For heterogeneous values.
import dik

var myDik = {"key0": %*{"foo": 42, "bar": 3.14}, "key1": %*["baz", true]}.toDik

doAssert myDik.len == 2                        # Length
doAssert myDik.cap == 2                        # Capacity
doAssert sizeOf(myDik) == 32

doAssert myDik[1].get == %*["baz", true]       # Get by index
doAssert myDik[^1].get == %*["baz", true]      # Get by BackwardsIndex
doAssert myDik["key1"].get == %*["baz", true]  # Get by key
doAssert myDik[1..1][0].get == %*["baz", true] # Get by Slice[int]

echo myDik.pretty                              # Pretty-print
doAssert myDik.toSeq is seq[Option[JsonNode]]  # Dik to seq
doAssert "key0" in myDik                       # contains(key)
myDik.del "key1"                               # Delete an item

for (index, key, value) in myDik.enumerated:   # Iterators
  doAssert index == 0
  doAssert key == "key0"
  doAssert value.get == %*{"foo": 42, "bar": 3.14}

# Can store the lack of a value, without using "nil".
myDik["key1"] = none JsonNode
doAssert not(myDik[1].isSome)
doAssert myDik[1] == myDik["key1"]

clear myDik
doAssert myDik.len == 0
doAssert $myDik == "{:}"

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