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haskell-dangerous-functions

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Documentation about Haskell's dangerous functions and a hlint config file to warn about them

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# Haskell's Dangerous Functions

## What does dangerous mean?

Dangerous could mean either of these:

- Partial: can throw exceptions in pure code
- Unsafe: can cause segfaults
- Has unexpected performance characteristics
- Doesn't do what you want
- Doesn't do what you think it does

## How to forbid these dangerous functions in your codebase

1. Copy the `hlint.yaml` file in this repository to `.hlint.yaml` within your repository

```
    cat /path/to/haskell-dangerous-functions >> /path/to/your/project/.hlint.yaml
    ```

2. Run `hlint` on your code.
   Make sure to require new changes to be `hlint`-clean.
   You can use `hlint --default` to generate a settings file ignoring all the hints currently outstanding.
   You can use [pre-commit hooks](https://pre-commit.com/) to forbid committing non-`hlint`-clean changes.

3. Whenever you want to make an exception, and use a forbidden function anyway, use the `ignore` key to add an exception to the `.hlint.yaml` file.

## FAQ

* **It seems pretty silly that these functions still exist, and their dangers not well-documented.**

I know! See [the relevant discussion on the GHC issue tracker](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5206).

* **Surely everyone knows about these?**

Maybe, but I certainly didn't, until they caused real production issues.

## Contributing

**WANTED: Evidence of the danger in these functions.**
If you can showcase a public incident with real-world consequences that happened because of one of these functions, I would love to refer to it in this document!

If you know about another dangerous function that should be avoided, feel free to submit a PR!
Please include:

* an `hlint` config to forbid the function in [`hlint.yaml`](hlint.yaml).
* a section in this document with:
  * Why the function is dangerous
  * A reproducible way of showing that it is dangerous.
  * An alternative to the dangerous function

It might be that the function you have in mind is not dangerous but still weird.
In that case you can add it to [the Haskell WAT list](https://github.com/NorfairKing/haskell-WAT).

## Overview of the dangerous functions

### [`forkIO`](https://hackage.haskell.org/package/base-4.14.1.0/docs/Control-Concurrent.html#v:forkIO)

TL;DR: Using `forkIO` is _VERY_ hard to get right, use the async library instead.

The main issue is that when threads spawned using `forkIO` throw an exception, this exception is not rethrown in the thread that spawned that thread.

As an example, suppose we `forkIO` a server and something goes wrong.
The main thread will not notice that anything went wrong.
The only indication that an exception was thrown will be that something is printed on `stderr`.

``` haskell
$ cat test.hs
#!/usr/bin/env stack
-- stack --resolver lts-15.15 script
{-# LANGUAGE NumericUnderscores #-}
import Control.Concurrent
main :: IO ()
main = do
  putStrLn "Starting our 'server'."
  forkIO $ do
    putStrLn "Serving..."
    threadDelay 1_000_000
    putStrLn "Oh no, about to crash!"
    threadDelay 1_000_000
    putStrLn "Aaaargh"
    undefined
  threadDelay 5_000_000
  putStrLn "Still running, eventhough we crashed"
  threadDelay 5_000_000
  putStrLn "Ok that's enough of that, stopping here."
```

Which outputs:

```
$ ./test.hs
Starting our 'server'.
Serving...
Oh no, about to crash!
Aaaargh
test.hs: Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:80:14 in base:GHC.Err
  undefined, called at /home/syd/test/test.hs:17:5 in main:Main
Still running, eventhough we crashed
Ok that's enough of that, stopping here.
```

Instead, we can use [`concurrently_` from the `async` package](http://hackage.haskell.org/package/async-2.2.3/docs/Control-Concurrent-Async.html#v:concurrently_):

``` haskell
$ cat test.hs
-- stack --resolver lts-15.15 script

{-# LANGUAGE NumericUnderscores #-}

import Control.Concurrent
import Control.Concurrent.Async

main :: IO ()
main = do
  putStrLn "Starting our 'server'."
  let runServer = do
        putStrLn "Serving..."
        threadDelay 1_000_000
        putStrLn "Oh no, about to crash!"
        threadDelay 1_000_000
        putStrLn "Aaaargh"
        undefined
  let mainThread = do
        threadDelay 5_000_000
        putStrLn "Still running, eventhough we crashed"
        threadDelay 5_000_000
        putStrLn "Ok that's enough of that, stopping here."
  concurrently_ runServer mainThread
```

to output:

See also:

* https://github.com/informatikr/hedis/issues/165
* https://github.com/hreinhardt/amqp/issues/96

### [`forkProcess`](https://hackage.haskell.org/package/unix-2.7.2.2/docs/System-Posix-Process.html#v:forkProcess)

Mostly impossible to get right.
You probably want to be using [the `async` library](http://hackage.haskell.org/package/async) instead.

If you think "I know what I'm doing" then you're probably still wrong.
Rethink what you're doing entirely.

See also https://www.reddit.com/r/haskell/comments/jsap9r/how_dangerous_is_forkprocess/

### Partial functions

#### [`head`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:head)

Throws an exception in pure code when the input is an empty list.

```
Prelude> head []
*** Exception: Prelude.head: empty list
```

Use `listToMaybe` instead.

Applies to [Data.Text.head](https://hackage.haskell.org/package/text-1.2.3.2/docs/Data-Text.html#v:head) as well

Trail of destruction:

* [`Prelude.head: empty list` makes hls unusable until restart](https://github.com/haskell/haskell-language-server/issues/1618)

#### [`tail`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:tail)

Throws an exception in pure code when the input is an empty list.

```
Prelude> tail []
*** Exception: Prelude.tail: empty list
```

Use `drop 1` or a case-match instead.

Applies to [Data.Text.tail](https://hackage.haskell.org/package/text-1.2.3.2/docs/Data-Text.html#v:tail) as well

#### [`init`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:init)

Throws an exception in pure code when the input is an empty list.

```
Prelude> init []
*** Exception: Prelude.init: empty list
```

Use a case-match on the `reverse` of the list instead, but keep in mind that it uses linear time in the length of the list.
Use a different data structure if that is an issue for you. 
Since `base-4.19` you can also employ [`unsnoc`](https://hackage.haskell.org/package/base/docs/Data-List.html#v:unsnoc).

Applies to [Data.Text.init](https://hackage.haskell.org/package/text-1.2.3.2/docs/Data-Text.html#v:init) as well

#### [`last`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:last)

Throws an exception in pure code when the input is an empty list.

```
Prelude> last []
*** Exception: Prelude.last: empty list
```

Use a `listToMaybe . reverse` instead, but keep in mind that it uses linear time in the length of the list.
Use a different data structure if that is an issue for you.
Since `base-4.19` you can also employ [`unsnoc`](https://hackage.haskell.org/package/base/docs/Data-List.html#v:unsnoc).

Applies to [Data.Text.last](https://hackage.haskell.org/package/text-1.2.3.2/docs/Data-Text.html#v:last) as well

#### [`'!!'`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:-33--33-)

Throws an exception in pure code when the index is out of bounds.

```
Prelude> [1, 2, 3] !! 3
*** Exception: Prelude.!!: index too large
```

It also allows negative indices, for which it also throws.

```
Prelude> [1,2,3] !! (-1)
*** Exception: Prelude.!!: negative index
```

The right way to index is to not use a list, because list indexing takes `O(n)` time, even if you find a safe way to do it.
If you _really_ need to deal with _list_ indexing (you don't), then you can use a combination of [`take`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:take) and [`drop`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:drop)
or (since `base-4.19`) [`'!?'`](https://hackage.haskell.org/package/base/docs/Data-List.html#v:-33--63-).

#### [`fromJust`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-Maybe.html#v:fromJust)

Throws an exception _in pure code_ when the input is `Nothing`.

```
Prelude Data.Maybe> fromJust Nothing
*** Exception: Maybe.fromJust: Nothing
CallStack (from HasCallStack):
  error, called at libraries/base/Data/Maybe.hs:148:21 in base:Data.Maybe
  fromJust, called at <interactive>:11:1 in interactive:Ghci1
```

Use a case-match instead.

#### [`read`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Text-Read.html#v:read)

There are multiple reasons not to use `read`.
The most obvious one is that it is partial.
It throws an exception _in pure code_ whenever the input cannot be parsed (and doesn't even give a helpful parse error):

```
Prelude> read "a" :: Int
*** Exception: Prelude.read: no parse
```

You can use `readMaybe` to get around this issue, HOWEVER:

The second reason not to use `read` is that it operates on `String`.

``` haskell
read :: Read a => String -> a
```

If you are doing any parsing, you should be using a more appropriate data type to parse: (`Text` or `ByteString`)

The third reason is that `read` comes from [the `Read` type class](https://hackage.haskell.org/package/base/docs/Text-Read.html#t:Read), which has no well-defined semantics.
In an ideal case, `read` and `show` would be inverses but this is _just not the reality_.
See [`UTCTime`](https://hackage.haskell.org/package/time/docs/Data-Time-Clock.html#t:UTCTime) as an example.

#### [`toEnum`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:toEnum)

The `toEnum :: Enum => Int -> a` function is partial whenever the `Enum`erable type `a` is smaller than `Int`:

```
Prelude> toEnum 5 :: Bool
*** Exception: Prelude.Enum.Bool.toEnum: bad argument
Prelude Data.Word> toEnum 300 :: Word8
*** Exception: Enum.toEnum{Word8}: tag (300) is outside of bounds (0,255)
```

#### [`succ`](https://hackage.haskell.org/package/base-4.15.0.0/docs/GHC-Enum.html#v:succ) and [`pred`](https://hackage.haskell.org/package/base-4.15.0.0/docs/GHC-Enum.html#v:pred)

These are partial, on purpose.
According to the docs:

> The calls `succ maxBound` and `pred minBound` should result in a runtime error.

```
Prelude Data.Word> succ 255 :: Word8
*** Exception: Enum.succ{Word8}: tried to take `succ' of maxBound
Prelude Data.Word> pred 0 :: Word8
*** Exception: Enum.pred{Word8}: tried to take `pred' of minBound
```

Use something like [`succMay`](https://hackage.haskell.org/package/safe-0.3.19/docs/Safe.html#v:succMay).

#### Functions involving division

* [`quot`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:quot)
* [`div`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:div)
* [`rem`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:rem)
* [`mod`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:mod)
* [`divMod`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:divMod)
* [`quotRem`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:quotRem)

```
Prelude> quot 1 0
*** Exception: divide by zero
Prelude> minBound `quot` (-1) :: Int
*** Exception: arithmetic overflow
Prelude> div 1 0
*** Exception: divide by zero
Prelude> minBound `div` (-1) :: Int
*** Exception: arithmetic overflow
Prelude> rem 1 0
*** Exception: divide by zero
Prelude> mod 1 0
*** Exception: divide by zero
Prelude> divMod 1 0
*** Exception: divide by zero
Prelude> quotRem 1 0
*** Exception: divide by zero
```

Whenever you consider using division, _really_ ask yourself whether you need division.
For example, you can (almost always) replace ``a `div` 2 <= b`` by `a <= 2 * b`.
(If you're worried about overflow, then use a bigger type.)

If your use-case has a fixed (non-`0`) _literal_ denominator, like ``a `div` 2``, and you have already considered using something other than division, then your case constitutes an acceptable exception.

Note that integer division may not be what you want in the first place anyway:

```
Prelude> 5 `div` 2
2 -- Not 2.5
```

See also https://github.com/NorfairKing/haskell-WAT#num-int

#### [`minimum`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:minimum) and [`maximum`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:maximum)

These functions throw an exception in pure code whenever the input is empty:

```
Prelude> minimum []
*** Exception: Prelude.minimum: empty list
Prelude> maximum []
*** Exception: Prelude.maximum: empty list
Prelude> minimum Nothing
*** Exception: minimum: empty structure
Prelude> minimum (Left "wut")
*** Exception: minimum: empty structure
Prelude Data.Functor.Const> minimum (Const 5 :: Const Int ())
*** Exception: minimum: empty structure
```

The same goes for [`minimumBy`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:minimumBy) and [`maximumBy`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Data-List.html#v:maximumBy).

You can use [`minimumMay`](http://hackage.haskell.org/package/safe-0.3.19/docs/Safe.html#v:minimumMay) from the [`safe`](http://hackage.haskell.org/package/safe) package (or a case-match on the `sort`-ed version of your list, if you don't want an extra dependency).

Applies to [Data.Text.maximum](https://hackage.haskell.org/package/text-1.2.3.2/docs/Data-Text.html#v:maximum) and [Data.Text.minimum](https://hackage.haskell.org/package/text-1.2.3.2/docs/Data-Text.html#v:minimum) as well

#### [`Data.Text.Encoding.decodeUtf8`](http://hackage.haskell.org/package/text-1.2.4.1/docs/Data-Text-Encoding.html#v:decodeUtf8)

Throws on invalid UTF-8 datao use `Data.Text.Encoding.decodeUtf8'` instead.

### Functions that throw exceptions in pure code on purpose

#### [`throw`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Control-Exception.html#v:throw)

Purposely throws an exception _in pure code_.

```
Prelude Control.Exception> throw $ ErrorCall "here be a problem"
*** Exception: here be a problem
```

Don't throw from pure code, use throwIO instead.

#### [`undefined`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:undefined)

Purposely fails, with a particularly unhelpful error message.

```
Prelude> undefined
*** Exception: Prelude.undefined
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:80:14 in base:GHC.Err
  undefined, called at <interactive>:1:1 in interactive:Ghci1
```

Deal with errors appropriately instead.

Also see `error` below.

#### [`error`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:error)

Purposely fails, with an only slightly less unhelpful error message than `undefined`.

```
Prelude> error "here be a problem"
*** Exception: here be a problem
CallStack (from HasCallStack):
  error, called at <interactive>:4:1 in interactive:Ghci1
```

Deal with errors appropriately instead.

If you're _really very extra sure_ that a certain case will never happen.
Bubble up the error to the `IO` part of your code and then use `throwIO` or `die`.

### Functions that do unexpected things

#### [`realToFrac`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:realToFrac)

This function goes through `Rational`:

```
-- | general coercion to fractional types
realToFrac :: (Real a, Fractional b) => a -> b
realToFrac = fromRational . toRational
```

`Rational` does not have all the values that a `Real` like `Double` might have, so things will go wrong in ways that you don't expect:

```
Prelude> realToFrac nan :: Double
-Infinity
```

Avoid general coercion functions and anything to do with `Double` in particular.

See also https://github.com/NorfairKing/haskell-WAT#real-double

#### [`%`](https://hackage.haskell.org/package/base-4.16.1.0/docs/Data-Ratio.html#v:-37-): Rational values

The `%` function is used to construct rational values:

``` haskell
data Ratio a = !a :% !a  deriving Eq
```
``` haskell
Prelude Data.Int Data.Ratio> 1 % 12 :: Ratio Int8
1 % 12
```

There are constraints on the two values in Rational values:

Recall (from the docs); "The numerator and denominator have no common factor and the denominator is positive."

When using fixed-size underlying types, you can end up with invalid `Ratio` values using `Num` functions:

```
Prelude Data.Int Data.Ratio> let r = 1 % 12 :: Ratio Int8
Prelude Data.Int Data.Ratio> r - r
0 % (-1)
Prelude Data.Int Data.Ratio> r + r
3 % (-14)
> r * r
1 % (-112)
```

When using arbitrarily-sized underlying types, you can end up with arbitrary runtime:

```
(1 % 100)^10^10^10 :: Rational -- Prepare a way to kill this before you try it out.
```

`Ratio` values create issues for any underlying type, so avoid them.
Consider whether you really need any rational values.
If you _really_ do, and you have a clear maximum value, consider using fixed-point values.
If that does not fit your use-case, consider using `Double` with all its caveats.

#### [`fromIntegral`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:fromIntegral) and [`fromInteger`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:fromInteger)

`fromIntegral` has no constraints on the size of the output type, so that output type could be smaller than the input type.
In such a case, it performs silent truncation:
```
> fromIntegral (300 :: Word) :: Word8
44
```
Similarly for `fromInteger`:
```
> fromInteger 300 :: Word8
44
```

`fromIntegral` has also had some very nasty bugs that involved the function behaving differently (even partially) depending on optimisation levels.
See [GHC #20066](https://gitlab.haskell.org/ghc/ghc/-/issues/20066) and [GHC #19345](https://gitlab.haskell.org/ghc/ghc/-/issues/19345).

Avoid general coercion functions but write specific ones instead, as long as the type of the result is bigger than the type of the input.

```
word32ToWord64 :: Word32 -> Word64
word32ToWord64 = fromIntegral -- Safe because Word64 is bigger than Word32
```

Prefer to use functions with non-parametric types and/or functions that fail loudly, like these:

* [`naturalToInteger :: Natural -> Integer`](https://hackage.haskell.org/package/base-4.15.0.0/docs/GHC-Natural.html#v:naturalToInteger)
* [`naturalToWord :: Natural -> Maybe Word`](https://hackage.haskell.org/package/base-4.15.0.0/docs/GHC-Natural.html#v:naturalToWordMaybe)
* [`toIntegralSized`](http://hackage.haskell.org/package/base-4.14.0.0/docs/Data-Bits.html#v:toIntegralSized)

Witness the trail of destruction:

* [Bug in `System.IO.hWaitForInput`](http://haskell.1045720.n5.nabble.com/Deprecating-fromIntegral-tt5864578.html) because of `fromIntegral`
* [Bug in cryptography-related code](https://github.com/haskell-crypto/cryptonite/issues/330) because of `fromIntegral`

I was also pointed to the [`finitary`](http://hackage.haskell.org/package/finitary) package but I haven't used it yet.

#### [`toEnum`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:toEnum)

The `toEnum` function suffers from the following problem **on top of being partial**.

Some instances of `Enum` use "the next constructor" as the next element while others use a `n+1` variant:

```
Prelude> toEnum 5 :: Double
5.0
Prelude Data.Fixed> toEnum 5 :: Micro
0.000005
```

Depending on what you expected, one of those doesn't do what you think it does.

#### [`fromEnum`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html#v:fromEnum)

From the docs:

> It is implementation-dependent what fromEnum returns when applied to a value that is too large to fit in an Int.

For example, some `Integer` that does not fit in an `Int` will be mapped to `0`, some will be mapped all over the place

```
Prelude> fromEnum (2^66 :: Integer) -- To 0
0
Prelude> fromEnum (2^65 :: Integer) -- To 0
0
Prelude> fromEnum (2^64 :: Integer) -- To 0
0
Prelude> fromEnum (2^64 -1 :: Integer) -- To -1 ?!
0
Prelude> fromEnum (2^63 :: Integer) -- To -2^63 ?!
-9223372036854775808
```

This is because `fromEnum :: Integer -> Int` is implemented using [`integerToInt`](https://hackage.haskell.org/package/integer-gmp-0.5.1.0/docs/GHC-Integer.html) which treats big integers and small integers differently.

#### [`succ`](https://hackage.haskell.org/package/base-4.15.0.0/docs/GHC-Enum.html#v:succ) and [`pred`](https://hackage.haskell.org/package/base-4.15.0.0/docs/GHC-Enum.html#v:pred)

These suffer from the same problem as `toEnum` (see above) **on top of being partial**.

```
Prelude> succ 5 :: Double
6.0
Prelude Data.Fixed> succ 5 :: Micro
5.000001
Prelude> pred 0 :: Word
*** Exception: Enum.pred{Word}: tried to take `pred' of minBound
Prelude Data.Ord Data.Int> succ (127 :: Int8)
*** Exception: Enum.succ{Int8}: tried to take `succ' of maxBound
```

#### `fromString` on `ByteString`

When converting to `ByteString`, `fromString` silently truncates to the bottom eight bits, turning your string into garbage.

```haskell
> print "⚠"
"\9888"
> print (fromString "⚠" :: ByteString)
"\160"
```

### The [`enumFromTo`](https://hackage.haskell.org/package/base-4.15.0.0/docs/GHC-Enum.html#v:enumFromTo)-related functions

These also suffer from the same problem as `toEnum` (see above)

```
Prelude> succ 5 :: Int
6
Prelude Data.Fixed> succ 5 :: Micro
5.000001
```

### Functions related to `String`-based IO

#### Input

- `System.IO.getChar`
- `System.IO.getLine`
- `System.IO.getContents`
- `System.IO.interact`
- `System.IO.readIO`
- `System.IO.readLn`
- `System.IO.readFile`

These behave differently depending on env vars, and actually fail on non-text data in files:

```
Prelude> readFile "example.dat"
*** Exception: Test/A.txt: hGetContents: invalid argument (invalid byte sequence) "\226\8364
```

See also [this blogpost](https://www.snoyman.com/blog/2016/12/beware-of-readfile/).

Use `ByteString`-based input and then use `Data.Text.Encoding.decodeUtf8'` if necessary. (But not `Data.Text.Encoding.decodeUtf8`, see above.)

#### Output

- `System.IO.putChar`
- `System.IO.putStr`
- `System.IO.putStrLn`
- `System.IO.print`
- `System.IO.writeFile`
- `System.IO.appendFile`

These behave differently depending on env vars:

```
$ ghci
Prelude> putStrLn "\973"
ύ
```

but
```
$ export LANG=C
$ export LC_ALL=C
$ ghci
Prelude> putStrLn "\973"
?
```

Use `ByteString`-based output, on encoded `Text` values or directly on bytestrings instead.

`writeFile` caused a real-world outage for @tomjaguarpaw on 2021-09-24.

See also [this blogpost](https://www.snoyman.com/blog/2016/12/beware-of-readfile/).

### Functions related to `Text`-based IO

- `Data.Text.IO.readFile`
- `Data.Text.IO.Lazy.readFile`

These have the same issues as `readFile`.

See also [this blogpost](https://www.snoyman.com/blog/2016/12/beware-of-readfile/).

Since `text-2.1` one can replace `Data.Text.IO` with [`Data.Text.IO.Utf8`](https://hackage.haskell.org/package/text-2.1/docs/Data-Text-IO-Utf8.html).

### Functions with unexpected performance characteristics

#### `nub`

`O(n^2)`, use [`ordNub`](https://github.com/nh2/haskell-ordnub) instead

Trail of destruction:
https://gitlab.haskell.org/ghc/ghc/-/issues/8173#note_236901

#### `foldl` and `foldMap`

Lazy. Use `foldl'` and `foldMap'` instead.

See [this excellent explanation](https://github.com/hasura/graphql-engine/pull/2933#discussion_r328821960).

#### `sum` and `product`
Lazy accumulator, but is fixed as of [GHC 9.0.1](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4675).

#### `genericLength`

`genericLength` consumes O(n) stack space when returning a strict numeric type.  Lazy numeric types (e.g. `data Nat = Z | S Nat`) are *very* rare in practice so `genericLength` is probably not what you want.

### Confusing functions

These functions are a _bad idea_ for no other reason than readability.
If there is a bug that involves these functions, it will be really easy to read over them.

#### `unless`

`unless` is defined as follows:

```
unless p s        =  if p then pure () else s
```

This is really confusing in practice use [`when`](https://hackage.haskell.org/package/base-4.15.0.0/docs/Control-Monad.html#v:when) with `not` instead.

#### `either`

Either takes two functions as arguments, one for the `Left` case and one for the `Right` case.
Which comes first? I don't know either, just use a case-match instead.

### Modules or packages to avoid

These are debatable, but requiring a good justification for using them is a good default.

#### [`Control.Lens`](https://hackage.haskell.org/package/lens)

The `lens` package is full of abstract nonsense and obscure operators.
There are good reasons (in exceptional cases) for using it, like in [`cursor`](https://github.com/NorfairKing/cursor), for example, but it should be avoided in general.

It also has an ENORMOUS dependency footprint.

If you need to use a dependency that uses lenses without the `lens` dependency, you can use `microlens` to stick with the (relatively) simple parts of lenses.
If you need to use a dependency that uses `lens`, go ahead and use `lens`, but stick with `view` (`^.`) and `set` (`.~`).

### Extensions to avoid

These are also debatable and low priority compared to the rest in this document, but still interesting to consider

#### [`{-# LANGUAGE DeriveAnyClass #-}`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/derive_any_class.html#extension-DeriveAnyClass)

Just use

``` haskell
data MyExample
instance MyClass MyExample
```

instead of

``` haskell
data MyExample
  deriving MyClass
```

[GHC (rather puzzlingly) gives the recommendation to turn on `DeriveAnyClass` even when that would lead to code that throws an exception in pure code at runtime.](https://gitlab.haskell.org/ghc/ghc/-/issues/19692)
As a result, banning this extension brings potentially great upside: preventing a runtime exception, as well as reducing confusion, for the cost of writing a separate line for an instance if you know what you are doing.

See also [this great explainer video by Tweag](https://www.youtube.com/watch?v=Zdne-Ch2000).

#### [`{-# LANGUAGE TupleSections #-}`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/tuple_sections.html)

This lets you add `{-# LANGUAGE TupleSections #-}` and potential confusion to write `(, v)` instead of `\a -> (a, v)`.

Whenever you feel the need to use `TupleSections`, you probably want to be using a data type instead of tuples instead.

#### [`{-# LANGUAGE DuplicateRecordFields #-}`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/duplicate_record_fields.html)

To keep things simple, use prefix-named record fields like this:

``` haskell
data Template = Template { templateName :: Text, templateContents :: Text }
```

instead of this

``` haskell
{-# LANGUAGE DuplicateRecordFields #-}
data Template = Template { name :: Text, contents :: Text }
```

It may be more typing but it makes code a lot more readable.

If you are concerned about not being able to auto-derive `aeson`'s `ToJSON` and `FromJSON` instances anymore,
you shouldn't be. You can still that using something like [`aeson-casing`](https://hackage.haskell.org/package/aeson-casing).
It's also dangerous to have serialisation output depend on the naming of things in your code, so be sure to test your serialisation with both property tests via [`genvalidity-sydtest-aeson`](https://hackage.haskell.org/package/genvalidity-sydtest-aeson) and golden tests via [`sydtest-aeson`](https://github.com/NorfairKing/sydtest).

#### [`{-# LANGUAGE NamedFieldPuns #-}`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/record_puns.html#extension-NamedFieldPuns)

Introduces unnecessary syntax.

For this example:
``` haskell
data C = C { a :: Int }
```

just use this:

``` haskell
f c = foo (a c)
```

instead of this:

``` haskell
f (C {a}) = foo a
```

#### [`{-# LANGUAGE OverloadedLabels #-}`](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/overloaded_labels.html#extension-OverloadedLabels)

If you're using this, you either know what you're doing - in which case you should know better than to use this - or you don't - in which case you definitely shouldn't use it.
Keep your code simple and just use record field selectors instead.

This extension often goes hand in hand with lens usage, which should also be discouraged, see above.

### Unsafe functions
#### `unsafePerformIO`

Before you use this function, first read [its documentation](http://hackage.haskell.org/package/base-4.14.1.0/docs/System-IO-Unsafe.html#v:unsafePerformIO) carefully.
If you've done (and I know you haven't, you liar) and still want to use it, read the following section first.

When you use `unsafePerformIO`, you pinkie-promise that the code in the `IO a` that you provide is definitely always 100% pure, you swear.
If this is not the case, all sorts of assumptions don't work anymore.
For example, if the code that you want to execute in `unsafePerformIO` is not evaluated, then the IO is never executed:

``` haskell
Prelude> fmap (const 'o') $ putStrLn "hi"
hi
'o'
Prelude System.IO.Unsafe> const 'o' $ unsafePerformIO $ putStrLn "hi"
'o'
```

Another issue is that pure code can be inlined whereas IO-based code cannot. When you pinkie-promise that your code is "morally" pure, you also promise that inlining it will not cause trouble.
This is not true in general:

```
Prelude System.IO.Unsafe> let a = unsafePerformIO $ putStrLn "hi"
Prelude System.IO.Unsafe> a
hi
()
Prelude System.IO.Unsafe> a
()
```

Lastly, this function is also not type-safe, as you can see here:

```
$ cat file.hs
```

``` haskell
import Data.IORef
import System.IO.Unsafe

test :: IORef [a]
test = unsafePerformIO $ newIORef []

main = do
  writeIORef test [42]
  bang <- readIORef test
  print $ map (\f -> f 5 6) (bang :: [Int -> Int -> Int])
```

```
$ runhaskell file.hs
[file.hs: internal error: stg_ap_pp_ret
    (GHC version 8.8.4 for x86_64_unknown_linux)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
[1]    13949 abort (core dumped)  runhaskell file.hs
```

#### [`unsafeDupablePerformIO`](http://hackage.haskell.org/package/base-4.14.1.0/docs/System-IO-Unsafe.html#v:unsafeDupablePerformIO)

Like `unsafePerformIO` but is even less safe.

#### [`unsafeInterleaveIO`](http://hackage.haskell.org/package/base-4.14.1.0/docs/System-IO-Unsafe.html#v:unsafeInterleaveIO)

Used to define lazy IO, which should be avoided.

#### [`unsafeFixIO`](http://hackage.haskell.org/package/base-4.14.1.0/docs/System-IO-Unsafe.html#v:unsafeFixIO)

Unsafe version of `fixIO`.

#### [`unsafeCoerce`](https://hackage.haskell.org/package/base-4.21.0.0/docs/Unsafe-Coerce.html#v:unsafeCoerce)

Can cause segfaults. All bets are off:

```
ghci> (unsafeCoerce (5 :: Int) :: (Int -> String)) 5
"[1]    3415638 segmentation fault (core dumped)  ghci
```

### Deprecated

#### `return`

Use `pure` instead.
See https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/monad-of-no-return

haskell-dangerous-functions

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