View source for Pseudo-function

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<blockquote>
Ever since McCarthy referred to the I/O operations in LISP 1.5 as "pseudo-functions"
functional I/O has been viewed with suspicion.

:<small>[https://web.archive.org/web/20240618013631/https://www.audentia-gestion.fr/research.microsoft/fpio.pdf Functional Programming and Input/Output], Andrew D. Gordon (page 146 of 168).</small>
</blockquote>

A pseudo-function (or <i>faux-function</i>) only <i>resembles</i> a Haskell
function - they usually have none of the benefits normally associated with
functions in Haskell, such as simpler
[[Equational reasoning examples|equational reasoning]] and
[[parallelism]]. But worst of all, the use of pseudo-functions in a Haskell
program means the types of its expressions, inferred or declared, can no
longer be relied on (for example, to show where externally visible side
effects are being used).

To then find a bug would involve a slow and laborious manual search through
most, if not all of the program's sources, along with that of any libraries
it relies on: not a practical option for codebases containing many million
lines of source code.

The use of pseudo-functions in Haskell should therefore be extensively
documented (in particular, why they're being used at all), or be
completely avoided.

<br><b>A rogues' gallery</b>

These are just some of the pseudo-functions lurking in Haskell libraries:

* [https://hackage.haskell.org/package/base-4.21.0.0/docs/Foreign-Marshal-Unsafe.html#v:unsafeLocalState <code>unsafeLocalState</code>]
* [https://hackage.haskell.org/package/base/docs/System-IO-Unsafe.html#v:unsafePerformIO <code>unsafePerformIO</code>]
* [https://hackage.haskell.org/package/text-2.1.2/docs/Data-Text-Unsafe.html#v:inlinePerformIO <code>inlinePerformIO</code>]
* [https://hackage.haskell.org/package/base/docs/System-IO-Unsafe.html#v:unsafeInterleaveIO <code>unsafeInterleaveIO</code>]
* [https://hackage.haskell.org/package/base/docs/Control-Monad-ST-Lazy-Unsafe.html#v:unsafeInterleaveST <code>unsafeInterleaveST</code>]
* [https://hackage.haskell.org/package/base/docs/Control-Monad-ST-Lazy-Unsafe.html#v:unsafeIOToST <code>unsafeIOToST</code>]
* [https://hackage.haskell.org/package/base/docs/GHC-Conc.html#v:unsafeIOToSTM <code>unsafeIOToSTM</code>]
* [https://hackage.haskell.org/package/array/docs/Data-Array-Unsafe.html#v:unsafeFreeze <code>unsafeFreeze</code>]
* [https://hackage.haskell.org/package/array/docs/Data-Array-Unsafe.html#v:unsafeThaw <code>unsafeThaw</code>]
* [https://hackage.haskell.org/package/base/docs/Unsafe-Coerce.html#v:unsafeCoerce-35- <code>unsafeCoerce#</code>]

<b>See also</b>

* [https://hackage.haskell.org/package/bytestring/docs/Data-ByteString-Internal.html#v:accursedUnutterablePerformIO <code>accursedUnutterablePerformIO</code>]
:* [http://www.reddit.com/r/haskell/comments/2cbgpz/flee_traveller_flee_or_you_will_be_corrupted_and/ Flee traveller! Flee or you will be corrupted and devoured!]


[[Category:Glossary]]