# Writing a PRNG using ChatGPT

I needed a pseudorandom number generator (PRNG) for a project I was working on. And Idris does come with that built-in: System.Random. Except, its functionality lives in the IO world and my project involved stuff working at the type-level, which doesn’t do IO (because having the compiler be able to arbitrary I/O is probably a bad idea). So I needed to write my own.

It didn’t need to be good, this was for a proof-of-concept, and I’d heard that ChatGPT was decent for simple single-use code, so I thought I might as well try it. It turned out pretty good:

[TH] Hello. I’m trying to write some Idris code for generating pseudorandom numbers. Could you help me with this?

[ChatGPT] Sure, I can help you with that! There are several ways to generate pseudorandom numbers in Idris, but one of the simplest is to use the standard library’s Random module.

Here’s a basic example of how you could use the Random module to generate a random integer between 0 and 99:

import Random

randInt : IO Int
randInt = do
gen <- mkStdGen 42 -- Seed the random number generator with a value of 42
rand <- randomR (0, 99) gen -- Generate a random integer between 0 and 99
pure rand


In this example, we’re using the mkStdGen function to create a new random number generator with a seed value of 42. We then use the randomR function to generate a random integer between 0 and 99. Finally, we return the generated random number using pure.

Note that the randInt function has an IO type, which means that it has side effects and cannot be pure. This is because generating random numbers is impure, since it relies on an external source of randomness.

If you want to generate a sequence of random numbers, you can use the RandomGen typeclass to create a random number generator and the random function to generate a random value from that generator:

Sensible first response, but it has the problem with using IO. Let’s try being more specific:

[TH] Ah but this requires using IO. I am trying to write a pure library, where all the functions will be implemented from scratch. Is this something you can help with?

[ChatGPT] Certainly! To generate pseudorandom numbers in a pure context, you’ll need to use a pure random number generator. One popular choice for this is the Mersenne Twister algorithm, which is known for its good statistical properties and long period.

Idris has a package for Mersenne Twister called mt that you can use to implement a pure random number generator.

Here’s an example of how you could use the mt package to generate a list of random integers:

module Random

import mt

generateRandomSequence : Int -> List Int
generateRandomSequence n = evalState (replicateM n random) (initGen 12345)

random : State MT Int
random = do
gen <- get
let (x, gen') = twist gen
put gen'
pure x


In this example, we define a function generateRandomSequence that takes an integer n and returns a list of n random integers. The function uses the evalState function from the standard library’s Control.Monad.State module to run a sequence of stateful actions in the MT monad.

The stateful action that generates a single random integer is defined by the random function, which uses the get function to get the current state of the random number generator, the twist function from the mt package to generate a new random value and a new state, and the put function to update the state of the random number generator.

The random number generator is initialized with a seed value of 12345 using the initGen function from the mt package.

Note that since the generateRandomSequence function is pure, it does not have any side effects and can be used in any context without causing impurity.

Ah… This is a common problem with ChatGPT: it will happily make up modules and APIs as need be. There is no mt module. But that’s okay, we can just ask it to help write something like that. At this point I had skimmed a paper on PRNGs, meaning I knew the words “Merkle-Damgård construction”, but not what they meant.

[TH] Okay, but we are trying to write the randomness function itself. Given that Idris represents 32-bits as Bits32 and 64-bits as Bits64, could you help me write a Merkel-Damgård construction in Idris?

[ChatGPT] Certainly! The Merkle-Damgard construction is a way to build a hash function from a compression function. To implement a pseudorandom number generator based on this construction, we can use the compression function to update the state of the generator and produce a new random value.

Here’s an example implementation of a 32-bit pseudorandom number generator based on the Merkle-Damgard construction:

module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = PRNGState Word32

-- Define the compression function
compress : Word32 -> Word32 -> Word32
compress state input = (state shiftL 1) xor input

-- Define the random function
random : PRNGState -> (Word32, PRNGState)
random (PRNGState state) =
let newState = compress state 0x6C078965
output = compress newState 0x7F4A7C13
in (output, PRNGState newState)


In this example, we define a data type PRNGState to represent the state of the pseudorandom number generator. The state is initialized with a 32-bit seed value.

We define the compress function to be the compression function used in the Merkle-Damgard construction. The compress function takes the current state of the generator and an input word, and produces a new state by left-shifting the current state by one bit and XORing it with the input word.

The random function takes a PRNGState value and produces a pair consisting of a random 32-bit word and a new PRNGState value. The function first uses the compress function to update the state of the generator with the constant value 0x6C078965. It then uses the compress function again to produce a random output word using the updated state and the constant value 0x7F4A7C13.

To generate a sequence of random numbers, you can use the randoms function from the standard library’s Data.List module, which takes a generator function and a count and returns a list of random values:

import Data.List

generateRandomSequence : Int -> List Word32
generateRandomSequence n = randoms (fst . random) (PRNGState 12345) n


In this example, we define a function generateRandomSequence that takes an integer n and returns a list of n random 32-bit words. The function uses the randoms function to generate the random sequence by applying the random function to a PRNGState value initialized with a seed value of 12345.

Now we’re getting somewhere! But where did those magic numbers come from?…

[TH] I’m not sure I understand how the constant values were picked. Could you please explain this?

[ChatGPT] Certainly! The constant values used in the compression function are crucial to the security of the pseudorandom number generator.

In the case of the Merkle-Damgard construction, the compression function takes two inputs: the current state of the hash function, and a new message block that needs to be hashed. The job of the compression function is to mix these inputs together in a way that makes it hard for an attacker to predict the output of the hash function.

To achieve this, the compression function should be designed to have good diffusion and confusion properties. Diffusion means that a small change in the input should result in a significant change in the output, while confusion means that the output should not reveal any information about the input.

The constant values used in the compression function are carefully chosen to achieve these properties. In the case of the 32-bit pseudorandom number generator example I provided, the constant values are:

• 0x6C078965
• 0x7F4A7C13

These values were chosen based on the “Mersenne Twister” pseudorandom number generator, which is a widely-used and well-studied algorithm based on the same Merkle-Damgard construction. The specific values were chosen based on a combination of empirical testing and mathematical analysis, to ensure that the resulting pseudorandom number generator has good statistical properties and is resistant to attacks.

In practice, the choice of constants can be an area of ongoing research and analysis for cryptographic algorithms. It’s important to choose constants that are well-vetted and have been shown to provide good security properties, and to update the constants periodically as new research and attacks are discovered.

Neat! Now to correct the syntax errors:

[TH] Okay, I see now, thanks for the explanation! In Idris, we use Bits32 not Word32. Could you please update the code to reflect this?

[ChatGPT] Certainly! Here is the updated code that uses Bits32 instead of Word32:

module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = PRNGState Bits32

-- Define the compression function
compress : Bits32 -> Bits32 -> Bits32
compress state input = (state shl 1) xor input

-- Define the random function
random : PRNGState -> (Bits32, PRNGState)
random (PRNGState state) =
let newState = compress state 0x6C078965
output = compress newState 0x7F4A7C13
in (output, PRNGState newState)


In this updated version, we define the PRNGState data type to hold a 32-bit value as a Bits32. The compress function takes two Bits32 values as input and produces a new Bits32 value as output. The compress function is defined to left-shift the state value by one bit and XOR it with the input value.

The random function takes a PRNGState value and produces a pair consisting of a random 32-bit word as a Bits32 value and a new PRNGState value. The function first uses the compress function to update the state of the generator with the constant value 0x6C078965. It then uses the compress function again to produce a random output word using the updated state and the constant value 0x7F4A7C13.

Note that the shl function is used instead of shiftL to left-shift the state value by one bit, since shl operates on Bits32 values.

It’s not right about the shl function, it is actually just shiftL, but that’s a simple fix. Now, I would like to stick this in a repository, so I’ll need a README. I could write this myself, but hey we’re having fun with this anyway, so how far can we get?

[TH] Wonderful! And could you help me write a README file for the github repository? It should have a brief note on what this code is, mention that it is licensed under the BSD-3-Clause license, and a disclaimer explaining that ChatGPT helped write the code.

[ChatGPT] Of course! Here’s an example README file that you can use:

# Idris Pseudorandom Number Generator

This is a simple pseudorandom number generator written in the Idris programming language. The generator uses a Merkle-Damgard construction with a 32-bit compression function to


Fine, but I’d like to be more copy-pasteable:

[TH] Thanks. Could you write that same README in markdown format, using 80-character line limits?

[ChatGPT] Certainly! Here’s an updated version of the README in Markdown format with 80-character line limits:

# Idris Pseudorandom Number Generator

This is a simple pseudorandom number generator written in the Idris programming
language. The generator uses a Merkle-Damgard construction with a 32-bit
compression function to produce pseudorandom 32-bit values.

This code is licensed under the BSD-3-Clause license. See the LICENSE file for
details.

## Disclaimer

This code was written with the assistance of ChatGPT, a language model created by
OpenAI. The author of this repository takes full responsibility for the
implementation and any errors or omissions that may be present.


Now that’s nice! I’m not super comfortable with the phrase about taking full responsibility for the implementation and any errors though. This code should definitely not be used for anything serious!…

[TH] Could you please add a sentence to the disclaimer explaining that this library may not be robust and so shouldn’t be used for serious cryptographic work or anything where the randomness is critical?

[ChatGPT] Certainly! Here’s the updated disclaimer with the additional sentence:

## Disclaimer

This code was written with the assistance of ChatGPT, a language model created by
OpenAI. The author of this repository takes full responsibility for the
implementation and any errors or omissions that may be present. However, please
note that this library may not be robust and therefore should not be used for
serious cryptographic work or any applications where the quality of the
randomness is critical. Use at your own risk.


Much better! At this point, I had also tried pasting the code into an Idris file and as a result, had gotten a complaint from the compiler: PRNGState has the constructor PRNGState. ChatGPT is using Haskell syntax (which makes sense, given that there is more Haskell than Idris out there so Haskell’s syntax is the statistically more likely when generating the next word).

[TH] Thanks! I’ve spotted another mistake in the code: In Idris, we cannot use the name of the datatype as the name of the constructor. Instead, it is common to use ‘Mk’ as a prefix in the constructor. Could you please update the code?

[ChatGPT] Sure! Here is the updated code that uses the MkPRNGState constructor:

module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = MkPRNGState Bits32

-- Define the compression function
compress : Bits32 -> Bits32 -> Bits32
compress state input = (state shl 1) xor input

-- Define the random function
random : PRNGState -> (Bits32, PRNGState)
random (MkPRNGState state) =
let newState = compress state 0x6C078965
output = compress newState 0x7F4A7C13
in (output, MkPRNGState newState)


In this updated version, we use the MkPRNGState constructor to create values of the PRNGState type. The random function is updated to pattern-match against the MkPRNGState constructor.

Now that’s pretty cool, it also updated the calls and pattern-matches! But the compress function still looks magic to me:

[TH] How have you chosen the compress function in the code?

[ChatGPT] The compress function used in the code is a simple bitwise operation that takes two Bits32 values as inputs and returns a Bits32 value. The implementation is taken from the TinyMT pseudorandom number generator, which is a small, fast, and high-quality pseudorandom number generator designed for use in simulations and gaming applications.

The specific values used in the compress function were chosen based on the characteristics of the TinyMT generator. The 0x6C078965 constant is the default multiplier used in the TinyMT algorithm, and the 0x7F4A7C13 constant is used as a default XOR value. These values have been shown to produce good quality pseudorandom numbers with good statistical properties.

Right. ChatGPT is trained on existing text, including code, so in cases where there is a common way to implement a thing, it makes sense that it copied one of those. That should probably be cited:

[ChatGPT] Sure, here’s the updated code with comments added to explain where each implementation is taken from:

module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = MkPRNGState Bits32

-- Define the compression function
-- The implementation is taken from the TinyMT pseudorandom number generator
-- https://github.com/MersenneTwister-Lab/TinyMT/blob/master/TINYMT32.h
compress : Bits32 -> Bits32 -> Bits32
compress state input = (state shl 1) xor input

-- Define the random function
-- The implementation is a simple Merkle-Damgard construction that uses the
-- compress function to produce pseudorandom 32-bit values
random : PRNGState -> (Bits32, PRNGState)
random (MkPRNGState state) =
let newState = compress state 0x6C078965
output = compress newState 0x7F4A7C13
in (output, MkPRNGState newState)


In this updated version of the code, each implementation is now accompanied by a comment that explains where it was taken from.

Now here’s where I got over-excited and forgot what I was dealing with: that link is plausible and so I happily put it in the code. I have since reverted that commit because the link doesn’t actually point anywhere. The user and repo are real, but the path to the file is wrong. The correct link is:

https://github.com/MersenneTwister-Lab/TinyMT/blob/master/tinymt/tinymt32.h

However, this is still wrong, because it turns out the file doesn’t contain the constants. So the reference is made up, insofar as it doesn’t contain what ChatGPT claims it does, and where the magic constants actually are taken from is anyone’s guess…

Anyway, remember how I said shl doesn’t exist? Why refactor manually when you can ask ChatGPT to do so?

[TH] And another minor fix: In Idris the function is actually shiftL not shl. Could you please fix this?

[ChatGPT] You’re correct, my apologies for the confusion. The correct function in Idris for performing a left shift is shiftL. Here is the updated code:

module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = MkPRNGState Bits32

-- Define the compression function
-- The implementation is taken from the TinyMT pseudorandom number generator
-- https://github.com/MersenneTwister-Lab/TinyMT/blob/master/TINYMT32.h
compress : Bits32 -> Bits32 -> Bits32
compress state input = (state shiftL 1) xor input

-- Define the random function
-- The implementation is a simple Merkle-Damgard construction that uses the
-- compress function to produce pseudorandom 32-bit values
random : PRNGState -> (Bits32, PRNGState)
random (MkPRNGState state) =
let newState = compress state 0x6C078965
output = compress newState 0x7F4A7C13
in (output, MkPRNGState newState)


Thanks for bringing that to my attention!

Here it hasn’t actually “learnt” anything, it has just taken my correction as the ground truth. Which is fine for corrections in programs, but it does assume the correction you’ve given ChatGPT is actually correct.

At this stage, I was enthusing with a friend about the generated results. I was thoroughly impressed that it could write Idris (subject to some corrections, but still), and even more so that the code actually seemed to work! My friend asked a great question: “If it can do this, what about Agda, which has mixfix operators?”.

I know very little Agda, and have never used it, so I didn’t know. But wait! Given that ChatGPT wrote the code, would it know? It definitely would have at least as good an understanding of Agda’s syntax as I did; probably better actually!

[TH] Would this code work in Agda with mixfix operators?

[ChatGPT] The Idris code we wrote should be translatable to Agda without much difficulty. However, Agda does not support mixfix operators in the same way that Idris does. Instead, you can define custom operator syntax using infix and postfix declarations.

Here is the equivalent Agda code for the Random module using infix declarations for the compress and random functions:


module Random where

open import Data.Bits
open import Data.Word

-- Define the state of the generator
record PRNGState : Set where
constructor MkPRNGState
field
state : Bits32

-- Define the compression function
-- The implementation is taken from the TinyMT pseudorandom number generator
-- https://github.com/MersenneTwister-Lab/TinyMT/blob/master/TINYMT32.h
_⊕_ : Bits32 → Bits32 → Bits32
state ⊕ input = (state ≪ 1) ⊕ input

-- Define the random function
-- The implementation is a simple Merkle-Damgard construction that uses the
-- compress function to produce pseudorandom 32-bit values
_⟨$⟩_ : PRNGState → (Bits32 × PRNGState) state ⟨$⟩ =
let newState = state.state ⊕ 0x6C078965
output = newState ⊕ 0x7F4A7C13
in (output , MkPRNGState newState)


In this version of the code, we’ve defined the compress function as an infix operator using the symbol ⊕. We’ve also defined the random function using infix syntax, using the ⟨\$⟩ symbol to indicate application of the function. Note that we’ve also changed the record syntax to match the syntax used in Agda.

Impressive! I couldn’t check it since I don’t have Agda installed, but I sent it back to my friend, who said it looked plausible! How cool is that??

To try to wrap things up, I asked it to add a note to the disclaimer about the fact that I am not a cryptography expert:

[TH] A final note for the diclaimer: I am not a cryptography expert. Could you please include this in the disclaimer?

[ChatGPT] Certainly! Here is an updated version of the disclaimer that includes the additional note:

DISCLAIMER: This library is provided as-is, without any express or implied warranty. The code may not be robust and should not be used for serious cryptographic work or anything where the randomness is critical. This code is licensed under the BSD-3-Clause license. Please note that I, ChatGPT, am not a cryptography expert, and this code should not be considered expert advice or opinion.


Please feel free to modify the disclaimer to better suit your needs.

[TH] Could you please reformat that to Markdown with 80 characters as the line limit?

[ChatGPT] Sure, here’s the updated disclaimer in Markdown format with 80 characters as the line limit:

DISCLAIMER: This library is provided as-is, without any express or implied
warranty. The code may not be robust and should not be used for serious
cryptographic work or anything where the randomness is critical. This code is
cryptography expert, and this code should not be considered expert advice or
opinion.


Let me know if you need any further assistance!

That wasn’t quite what I asked: it had specified that ChatGPT wasn’t a cryptography expert, not that I, Thomas, was not. And for some reason it seems to have lost the ability to use markdown formatting: ‘DISCLAIMER’ should be a heading. But it still works, I just need to reword it a tiny bit.

I tried to get it to improve my working version of the code, but it struggled with names, let-bindings, and where-blocks. It mostly ran in circles or changed the code when it was meant to only be renaming variables. It’s not super interesting to look at…

But if you’re interested, expand this to see the exchange.

[TH] Here is the completed code. Could you please extract the constants to variables with appropriate types and commens?

module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = MkPRNGState Bits32

-- Define the compression function
-- The implementation is taken from the TinyMT pseudorandom number generator
-- https://github.com/MersenneTwister-Lab/TinyMT/blob/master/TINYMT32.h
compress : Bits32 -> Bits32 -> Bits32
compress state input = (state shiftL 1) xor input

-- Define the random function
-- The implementation is a simple Merkle-Damgard construction that uses the
-- compress function to produce pseudorandom 32-bit values
random : PRNGState -> (Bits32, PRNGState)
random (MkPRNGState state) =
let newState = compress state 0x6C078965
output = compress newState 0x7F4A7C13
in (output, MkPRNGState newState)


[ChatGPT] Sure, here’s the updated code with constants extracted to variables with appropriate types and comments:


module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = MkPRNGState Bits32

-- Define the compression function
-- The implementation is taken from the TinyMT pseudorandom number generator
-- https://github.com/MersenneTwister-Lab/TinyMT/blob/master/TINYMT32.h
compress : Bits32 -> Bits32 -> Bits32
compress state input =
let
leftShiftAmt : Int
leftShiftAmt = 1
polynomial : Bits32
polynomial = 0x9908B0DF
in (state shiftL leftShiftAmt) xor input xor (state shiftL polynomial)

-- Define the random function
-- The implementation is a simple Merkle-Damgard construction that uses the
-- compress function to produce pseudorandom 32-bit values
random : PRNGState -> (Bits32, PRNGState)
random (MkPRNGState state) =
let
input1 : Bits32
input1 = 0x6C078965
input2 : Bits32
input2 = 0x7F4A7C13
newState : Bits32
newState = compress state input1
output : Bits32
output = compress newState input2
in (output, MkPRNGState newState)


Let me know if you need any further assistance!

Lua-formatted?? That’s just completely wrong, but the code itself seems fine. I would prefer it to use where-blocks though:

[TH] Could you please rework that to where blocks instead of let bindings?

[ChatGPT] Sure, here it is with the same code structured as where clauses instead of let bindings:


module Random

import Data.Bits
import Data.Word

-- Define the state of the generator
data PRNGState = MkPRNGState Bits32

-- Define the compression function
-- The implementation is taken from the TinyMT pseudorandom number generator
-- https://github.com/MersenneTwister-Lab/TinyMT/blob/master


Now it’s SQL-formatted. Seemingly ChatGPT has lost the context for what language we’re using and is just guessing. That’s interesting. I also interrupted this response since it was no longer using the code I had given it.

Rather than getting it to reformat my code, I decided to ask it to write the function I needed: generating a list of n random numbers. This it handled without any problems:

[TH] From the following code, can you please write a function which takes a number and generates a list of that many random numbers?

||| Simple pseudorandom number generator using a Merkle-Damgård construction
||| with a 32-bit compression function to produce pseudorandom 32-bit values.
|||
||| Written with the assistance of ChatGPT, please do not be use for serious
||| cryptographic work or any applications where the quality of the randomness
||| is critical. (See README for full disclaimer)
module Random.Pure.MerkleDamgard

import Data.Bits

||| The state of the generator
export
data PRNGState = MkPRNGState Bits32

||| Compression function taken from the TinyMT pseudorandom number generator.
|||
||| https://github.com/MersenneTwister-Lab/TinyMT/blob/master/tinymt/tinymt32.h
-- CHATGPT WRONG: https://github.com/MersenneTwister-Lab/TinyMT/blob/master/tinymt32.h
export
compress : Bits32 -> Bits32 -> Bits32
compress state input = (state shiftL 1) xor input

||| A simple Merkle-Damgard construction that uses the compress function to
||| produce pseudorandom 32-bit values.
export
random : PRNGState -> (Bits32, PRNGState)
random (MkPRNGState state) =
let newState = compress state 0x6C078965
output = compress newState 0x7F4A7C13
in (output, MkPRNGState newState)


[ChatGPT] Sure! Here’s an implementation of a function generateRandomList that takes a PRNGState and a count of how many random numbers to generate and produces a list of that many random numbers.

generateRandomList : PRNGState -> Int -> List Bits32
generateRandomList state count =
let (rand, newState) = random state
in if count <= 0
then []
else rand :: generateRandomList newState (count - 1)


You can use it like this:


let initState = MkPRNGState 123456789 in
let randomNumbers = generateRandomList initState 10 in
-- do something with randomNumbers


This will generate a list of 10 pseudorandom 32-bit values. You can change the count to generate more or fewer values.

It would be really nice if this could be total though.

[TH] Can you rewrite the function using a Nat instead of an Int, recursing on the Nat?

[ChatGPT] Certainly! Here’s the modified function that uses Nat and recurses on it:

generateRandoms : Nat -> PRNGState -> List Bits32
generateRandoms Z _ = []
generateRandoms (S n) state =
let (r, newState) = random state
in r :: generateRandoms n newState


This function pattern matches on the Nat argument, with the base case being Z, which returns an empty list. For the inductive case, the function generates a random number and then recursively calls itself with the remaining number of random numbers to generate, decrementing the Nat argument by one.

Isn’t that cool?? It not only understood the pattern-matching on Nats and how it affects the number, it even included the _ wildcard for the first case, and used a readable let-binding for the recursive one!

Going mad with power, I decided to try to teach it linearity. Unfortunately that didn’t work:

[TH] Thank you! Finally, in Idris we can use linearity to enforce that the state can only be used once. For example, we could write

newState : (1 state : PRNGState) -> PRNGState
newState state = let 1 newstate = update state in newstate


where the 1 state tells Idris that the argument must be linear, i.e. used exactly once. Can you rewrite the function using this knowledge to enforce the state being linear?

[ChatGPT] Yes, using linearity we can ensure that the state is used only once. Here is an updated implementation that enforces linearity:

nextRandom : (state : PRNGState ** (Bits32, PRNGState))
nextRandom = (state ** (x, newState state))
where
x : Bits32
(x, _) = random state

newState : (1 state : PRNGState) -> PRNGState
newState state = let 1 newstate = update state in newstate

update : (1 state : PRNGState) -> PRNGState
update state = let MkPRNGState newstate = newState state
in MkPRNGState (compress newstate 0x7F4A7C13)


In this implementation, state is first passed as an argument to nextRandom. The function then uses the state to compute a random number x and a new state using newState. The 1 state syntax indicates that the state argument must be used exactly once in the newState and update functions. This ensures that the state is never used again after the function has computed the new state. The updated state is returned as a part of the result tuple using the ** operator.

This is not only syntactically wrong, but also an infinite loop: newState calls update which calls newState etc. The totality checker immediately spotted this and complained.

Ah well, move on to something less cool but more practical:

[ChatGPT] Yes, I do. I can see the previous messages if you have any follow-up questions.

[TH] Wonderful! How could we use the random function to generate a floating-point number between 0 and 1?

[ChatGPT] We can use the random function to generate a 32-bit integer and then divide it by the maximum 32-bit unsigned integer value (i.e. 2^32 - 1). This will give us a floating-point number in the range [0, 1). Here’s an example implementation:

toFloat : PRNGState -> (Double, PRNGState)
toFloat state =
let (bits, newState) = random state
maxInt = 4294967295.0 -- maximum 32-bit unsigned integer value
in (toDouble bits / maxInt, newState)


Here, we first call random to get a 32-bit integer bits and a new state newState. We then divide bits by maxInt to get a floating-point number between 0 and 1 (not including 1), and return this value along with the new state. Note that we convert bits to a Double using the toDouble function, which is part of the Bits module in Idris.

It’s made up Bits.toDouble, but other than that it’s sensible code. And it even explained how it works, which I greatly appreciated! Time to tell it how to fix things:

[TH] We don’t have a toDouble function, instead you can just use cast. Also, could you please rename the function to randomDouble?

[ChatGPT] Sure, here’s the updated code:

-- Generate a random floating-point number in the range [0, 1)
randomDouble : PRNGState -> (Double, PRNGState)
randomDouble st =
let (x, st') = random st
r = cast (fromInteger (toInteger x) / (2.0 ** 32))
in (r, st')


In this implementation, we convert the 32-bit integer value produced by random to a Double using cast and divide it by 2.0 ** 32 to get a value in the range [0, 1). Note that we need to first convert the 32-bit integer to an Integer before converting it to a Double, as cast only works with exact types.

It got a number of things wrong. Most notably the application of cast and the existence of a Python-like power operator.

[TH] Slight confusion: I meant to use the cast function instead of the toDouble function in the previous code, but still keep the code intact. Can you please do that?

[ChatGPT] Sure, here’s the updated code using cast instead of toDouble:

randomDouble : PRNGState -> (Double, PRNGState)
randomDouble state =
let (bits, newState) = random state
in (cast bits / cast 4294967295.0, newState)


Note that 4294967295 is 2^32 - 1, which is the maximum value that can be represented by Bits32. By casting it to a Double, we can use it to normalize the 32-bit output of the random function to a value between 0 and 1.

The cast on the max integer value is excessive due to it being a Double-literal, but other than that this is much better.

Not having learnt my lesson, I tried to get it to refactor the code to be nicer (e.g. storing magic numbers in variables with descriptive names).

It didn’t work this time either. Expand if you must.

[TH] Great! And can you please put the magic number in its own variable in the let-binding?

[ChatGPT] Sure, here’s the updated code with the magic number in its own variable:

randomDouble : PRNGState -> (Double, PRNGState)
randomDouble state =
let (bits, state') = random state
magic = 4294967295.0 -- 2^32 - 1
in (cast bits / magic, state')


[TH] Can you give the magic number a more descriptive name please?

[ChatGPT] Sure, we can use a more descriptive name for the magic number, such as MAX_UINT32:

randomDouble : PRNGState -> (Double, PRNGState)
randomDouble (MkPRNGState state) =
let MAX_UINT32 : Double = cast UInt32 (2 ^ 32) - 1.0
(rand, new_state) = random state
in (cast Double rand / MAX_UINT32, new_state)


This makes the code more self-documenting and easier to read.

[TH] Ah but you changed the number to now involve the power operator. Could you please just keep the original number?

[ChatGPT] I apologize for the confusion. Here is the previous implementation with the original magic number:

randomDouble : PRNGState -> (Double, PRNGState)
randomDouble state =
let (int, newState) = random state
mantissa = int # 20
exp = 1023 + (31 - clz mantissa)
bits = exp shiftL 52 .|. (mantissa shiftL 32)
d = cast bits
in (d - 1.0, newState)


If you would like to use a more descriptive name for the magic number, you can use the following instead:

randomDouble : PRNGState -> (Double, PRNGState)
randomDouble state =
let (int, newState) = random state
mantissa = int # 20
exp = 1023 + (31 - clz mantissa)
bias = 0x3FF
bits = (exp - bias) shiftL 52 .|. (mantissa shiftL 32)
d = cast bits
in (d - 1.0, newState)


Here, we’ve introduced the variable bias with the value 0x3FF which represents the exponent bias for double-precision numbers.

Once again, back to practical things: lists of random doubles.

[TH] And finally, can you please define a function which takes a Nat and generates a list of random doubles, using the randomDouble function we just defined?

[ChatGPT] Sure! Here’s an implementation of randomDoubles that uses randomDouble to generate a list of random Double values:

randomDoubles : Nat -> PRNGState -> List Double
randomDoubles Z _ = []
randomDoubles (S n) state =
let (x, newState) = randomDouble state
in x :: randomDoubles n newState


The function takes a Nat argument indicating the number of random doubles to generate, and a PRNGState argument representing the current state of the pseudorandom number generator. It returns a list of Double values.

The implementation uses a recursive definition that stops when it reaches Z, and otherwise generates a random Double using randomDouble, and then conses it onto the rest of the list generated by a recursive call with the new state.

Now I have everything I need, and I barely had to write any code!!

I think this technology is super cool. Especially for things where you only need a proof-of-concept, one-time script, or even (as in this case) a reimplementation of something existing and well-known. As I made sure it put many times in various disclaimers, this shouldn’t be used for anything serious since I do not have the knowledge to confirm its correctness and/or robustness, but it saved me a lot of time and was a lot more fun than writing the code by hand.

For the syntax errors, a really interesting question is what would happen if someone hooked ChatGPT up to the type-checker so that it had access to the typing information and error messages as part of its output-generation? However, that sounds very much like a research project on its own, and I don’t need more of those… If anyone has a go at it though, please let me know what happens!

Finally, it costs nothing to be polite. ChatGPT was extremely helpful and patient after all:

[TH] This has been amazing! Thank you so much for your help!!

[ChatGPT] You’re very welcome! It was my pleasure to help. If you have any more questions in the future, don’t hesitate to ask.

And that concludes this little adventure. Thanks for reading, I hope it was interesting! : )

Expand for screenshots of the conversation. The text in this post is a transcript. ##### Thomas Ekström Hansen
###### PhD student in Computer Science

My interests include information visualisation, formal methods, and low-level programming.

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