https://wiki.haskell.org/index.php?title=Global_variables&feed=atom&action=history
Global variables - Revision history
2024-03-29T09:33:02Z
Revision history for this page on the wiki
MediaWiki 1.35.5
https://wiki.haskell.org/index.php?title=Global_variables&diff=36785&oldid=prev
ConradParker: update John Hughe's functional pearl Globals.ps URL
2010-09-17T01:25:35Z
<p>update John Hughe's functional pearl Globals.ps URL</p>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* John Hughes has a Functional Pearl which advocates the use of [[implicit parameters]] for global variables. He also mentions the unsafePerformIO hack and the use of a Reader monad transformer, none of which are totally satisfactory: [http://<ins class="diffchange diffchange-inline">citeseerx</ins>.<ins class="diffchange diffchange-inline">ist</ins>.<ins class="diffchange diffchange-inline">psu</ins>.<ins class="diffchange diffchange-inline">edu</ins>/<ins class="diffchange diffchange-inline">viewdoc</ins>/<ins class="diffchange diffchange-inline">download?doi=10</ins>.<ins class="diffchange diffchange-inline">1.1.23.145&rep=rep1&type=</ins>ps Global Variables in Haskell] (2004)</div></td>
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ConradParker
https://wiki.haskell.org/index.php?title=Global_variables&diff=26506&oldid=prev
Adrian Hey: Deleting link to irrelevant haskell cafe post. This is *not* an example of the use of the "unsafePerformIO hack" under discussion.
2009-02-17T13:40:30Z
<p>Deleting link to irrelevant haskell cafe post. This is *not* an example of the use of the "unsafePerformIO hack" under discussion.</p>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* John Hughes has a Functional Pearl which advocates the use of [[implicit parameters]] for global variables. He also mentions the unsafePerformIO hack and the use of a Reader monad transformer, none of which are totally satisfactory: [http://www.cs.chalmers.se/~rjmh/Globals.ps Global Variables in Haskell] (2004)</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* John Hughes has a Functional Pearl which advocates the use of [[implicit parameters]] for global variables. He also mentions the unsafePerformIO hack and the use of a Reader monad transformer, none of which are totally satisfactory: [http://www.cs.chalmers.se/~rjmh/Globals.ps Global Variables in Haskell] (2004)</div></td>
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Adrian Hey
https://wiki.haskell.org/index.php?title=Global_variables&diff=26324&oldid=prev
Lemming: Edsko de Vries in Haskell Cafe
2009-02-06T16:49:26Z
<p>Edsko de Vries in Haskell Cafe</p>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* John Hughes has a Functional Pearl which advocates the use of [[implicit parameters]] for global variables. He also mentions the unsafePerformIO hack and the use of a Reader monad transformer, none of which are totally satisfactory: [http://www.cs.chalmers.se/~rjmh/Globals.ps Global Variables in Haskell] (2004)</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* John Hughes has a Functional Pearl which advocates the use of [[implicit parameters]] for global variables. He also mentions the unsafePerformIO hack and the use of a Reader monad transformer, none of which are totally satisfactory: [http://www.cs.chalmers.se/~rjmh/Globals.ps Global Variables in Haskell] (2004)</div></td>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"></td>
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Lemming
https://wiki.haskell.org/index.php?title=Global_variables&diff=15085&oldid=prev
EricKow at 09:01, 16 August 2007
2007-08-16T09:01:09Z
<p></p>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* People have been known to use [[Top level mutable state]], otherwise known as the "unsafePerformIO hack".</div></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>* John Hughes has a Functional Pearl which <del class="diffchange diffchange-inline">seems to advocate</del> the use of [[implicit parameters]] for global variables. He also mentions the unsafePerformIO hack: [http://www.cs.chalmers.se/~rjmh/Globals.ps Global Variables in Haskell] (2004)</div></td>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div>* John Hughes has a Functional Pearl which <ins class="diffchange diffchange-inline">advocates</ins> the use of [[implicit parameters]] for global variables. He also mentions the unsafePerformIO hack<ins class="diffchange diffchange-inline"> and the use of a Reader monad transformer, none of which are totally satisfactory</ins>: [http://www.cs.chalmers.se/~rjmh/Globals.ps Global Variables in Haskell] (2004)</div></td>
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EricKow
https://wiki.haskell.org/index.php?title=Global_variables&diff=15084&oldid=prev
EricKow: + john hughes's paper
2007-08-16T08:59:09Z
<p>+ john hughes's paper</p>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #ffe49c; vertical-align: top; white-space: pre-wrap;"><div>People have been known to use [[Top level mutable state]], otherwise known as the "unsafePerformIO hack".</div></td>
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<td style="color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins class="diffchange diffchange-inline">* </ins>People have been known to use [[Top level mutable state]], otherwise known as the "unsafePerformIO hack".</div></td>
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EricKow
https://wiki.haskell.org/index.php?title=Global_variables&diff=13087&oldid=prev
Adrian Hey at 20:16, 18 May 2007
2007-05-18T20:16:16Z
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<td style="background-color: #f8f9fa; color: #202122; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>* Jules Bean in [http://www.haskell.org/pipermail/haskell-cafe/2007-May/025526.html Haskell Cafe]</div></td>
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Adrian Hey
https://wiki.haskell.org/index.php?title=Global_variables&diff=13084&oldid=prev
Lemming: text taken from Haskell Cafe
2007-05-18T12:54:59Z
<p>text taken from Haskell Cafe</p>
<p><b>New page</b></p><div>== Question ==<br />
Is there a simple and straightforward way to use global variables in Haskell?<br />
<br />
== Answer ==<br />
<br />
(Perhaps annoyingly) the answer to this question, like so many other<br />
frequently asked questions, is a question. "What are you trying to do?"<br />
<br />
The reason for this is that haskell's abstractions are different from<br />
those of the mainstream imperative languages, and the mapping isn't 1-1.<br />
So for a particular 'C' feature, there may be 3 or 4 haskell features<br />
which achieve similar effects, and the correct choice depends on the<br />
detailed context.<br />
<br />
So, in particular, all those tasks which you use global variables for in<br />
C, can be achieved in haskell, but which solution depends what you are<br />
trying to do:<br />
<br />
# If you have some globals which are constant, then just define them at the top level. No problem.<br />
#: <hask> pi = 3.14 </hask><br />
#: <hask> progname = "MyCoolApp" </hask><br />
# If you have a global environment, which various functions read from (and you might, for example, initialise from a configuration file) then you should thread that as a parameter to your functions (after having, very likely, set it up in your 'main' action). If the explicit parameter passing annoys you, then you can 'hide' it with a [[Monad]].<br />
# If you have a need to store some global state which various of your functions modify, then you have a design issue. This style of design is frowned upon in C as well! The correct solution in C is to pass a 'bundle' of appropriate parameters (you might call it an environment) to those functions which need it. In C++, perl or python you'd very likely make this bundle an object. In haskell this bundle will be a data value in some custom type; and using Monads you can 'hide the details' of passing it around, if you wish.<br />
<br />
== Source ==<br />
<br />
* Jules Bean in [http://www.haskell.org/pipermail/haskell-cafe/2007-May/025526.html Haskell Cafe]<br />
<br />
<br />
[[Category:FAQ]]</div>
Lemming