https://wiki.haskell.org/index.php?title=Upgrading_from_MTL_1_to_MTL_2&feed=atom&action=historyUpgrading from MTL 1 to MTL 2 - Revision history2024-03-29T08:04:44ZRevision history for this page on the wikiMediaWiki 1.35.5https://wiki.haskell.org/index.php?title=Upgrading_from_MTL_1_to_MTL_2&diff=64128&oldid=prevAtravers: Minor formatting changes2021-04-09T00:43:47Z<p>Minor formatting changes</p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 00:43, 9 April 2021</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>Many packages written for earlier versions of MTL build unchanged with version 2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with version 1 without some CPP directives. Here are the fixes for common messages:</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>Many packages written for earlier versions of MTL build unchanged with version 2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with version 1 without some CPP directives. Here are the fixes for common messages:</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>''Illegal instance declaration for Applicative (State a)''</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>''Illegal instance declaration for Applicative (State a)''</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>:This usually means that the instance you're defining is now defined in the [http://hackage.haskell.org/package/transformers transformers] package. You may wish to check it's the same, so you can just delete it.</div></td>
<td class="diff-marker"> </td>
<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>:This usually means that the instance you're defining is now defined in the [http://hackage.haskell.org/package/transformers transformers] package. You may wish to check it's the same, so you can just delete it.</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>''Could not deduce (Functor m) from the context (Monad m)<br/>arising from a use of fmap''</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>''Could not deduce (Functor m) from the context (Monad m)<br/>arising from a use of fmap''</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>:This will be a situation where you're using a monad constructed with a transformer as a Functor. You can replace <hask>fmap</hask> with <hask>liftM</hask> and the code will build with old and new versions of mtl.</div></td>
<td class="diff-marker"> </td>
<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>:This will be a situation where you're using a monad constructed with a transformer as a Functor. You can replace <hask>fmap</hask> with <hask>liftM</hask> and the code will build with old and new versions of mtl.</div></td>
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<td class="diff-marker"> </td>
<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>
<td class="diff-marker"> </td>
<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>''Could not deduce (Functor m) from the context (Monad m)<br/>arising from the superclasses of an instance declaration''</div></td>
<td class="diff-marker">+</td>
<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>''Could not deduce (Functor m) from the context (Monad m)<br/>arising from the superclasses of an instance declaration''</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>:This will be another situation where you're using a monad constructed with a transformer as a Functor. You can replace the <hask>Monad m</hask> with <hask>Functor m</hask> to make it work with mtl-2, or add a <hask>Functor m</hask> constraint to make it work with old and new versions of mtl.</div></td>
<td class="diff-marker"> </td>
<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>:This will be another situation where you're using a monad constructed with a transformer as a Functor. You can replace the <hask>Monad m</hask> with <hask>Functor m</hask> to make it work with mtl-2, or add a <hask>Functor m</hask> constraint to make it work with old and new versions of mtl.</div></td>
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<td class="diff-marker"> </td>
<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>
<td class="diff-marker"> </td>
<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>''Not in scope: data constructor State''</div></td>
<td class="diff-marker">+</td>
<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>''Not in scope: data constructor State''</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>:The <hask>State</hask> type is now a type synonym. You could replace the <hask>State</hask> data constructor with the <hask>state</hask> function.</div></td>
<td class="diff-marker"> </td>
<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>:The <hask>State</hask> type is now a type synonym. You could replace the <hask>State</hask> data constructor with the <hask>state</hask> function.</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 class="diff-marker">−</td>
<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>''Not in scope: runState''</div></td>
<td class="diff-marker">+</td>
<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>''Not in scope: runState''</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>:You probably imported <hask>State(..)</hask>, which won't work now that <hask>State</hask> is a type synonym. You need to import <hask>State</hask> and <hask>runState</hask>. (That will work with old versions of mtl too.)</div></td>
<td class="diff-marker"> </td>
<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>:You probably imported <hask>State(..)</hask>, which won't work now that <hask>State</hask> is a type synonym. You need to import <hask>State</hask> and <hask>runState</hask>. (That will work with old versions of mtl too.)</div></td>
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<td class="diff-marker"> </td>
<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>
<td class="diff-marker"> </td>
<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 class="diff-marker">−</td>
<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>''Illegal instance declaration for Myclass (State Foo)''</div></td>
<td class="diff-marker">+</td>
<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>''Illegal instance declaration for Myclass (State Foo)''</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>:If you have a matching instance for <hask>StateT</hask>, you can delete the instance for <hask>State</hask>. Otherwise you need to generalize your instance to <hask>StateT</hask>. If that's not possible, you may need to introduce a <hask>newtype</hask>.</div></td>
<td class="diff-marker"> </td>
<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>:If you have a matching instance for <hask>StateT</hask>, you can delete the instance for <hask>State</hask>. Otherwise you need to generalize your instance to <hask>StateT</hask>. If that's not possible, you may need to introduce a <hask>newtype</hask>.</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>
<td class="diff-marker"> </td>
<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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</table>Atravershttps://wiki.haskell.org/index.php?title=Upgrading_from_MTL_1_to_MTL_2&diff=38009&oldid=prevHowardBGolden: Spelling correction2010-12-25T06:14:10Z<p>Spelling correction</p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 06:14, 25 December 2010</td>
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<td colspan="2" class="diff-lineno">Line 1:</td>
<td colspan="2" class="diff-lineno">Line 1:</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>Version 2 of the [[Monad Transformer Library]] introduced some</div></td>
<td class="diff-marker"> </td>
<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>Version 2 of the [[Monad Transformer Library]] introduced some</div></td>
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<td class="diff-marker">−</td>
<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>[[Incompatibilities between MTL 1 and MTL 2|<del class="diffchange diffchange-inline">incompatibilites</del>]]</div></td>
<td class="diff-marker">+</td>
<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>[[Incompatibilities between MTL 1 and MTL 2|<ins class="diffchange diffchange-inline">incompatibilities</ins>]]</div></td>
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<td class="diff-marker"> </td>
<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>relative to version 1.</div></td>
<td class="diff-marker"> </td>
<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>relative to version 1.</div></td>
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<td class="diff-marker"> </td>
<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>This page provides instructions for upgrading code written for</div></td>
<td class="diff-marker"> </td>
<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>This page provides instructions for upgrading code written for</div></td>
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</table>HowardBGoldenhttps://wiki.haskell.org/index.php?title=Upgrading_from_MTL_1_to_MTL_2&diff=37466&oldid=prevYitzGale: Added another introductory paragraph, with links2010-11-16T11:45:30Z<p>Added another introductory paragraph, with links</p>
<table class="diff diff-contentalign-left diff-editfont-monospace" data-mw="interface">
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 11:45, 16 November 2010</td>
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<td colspan="2" class="diff-lineno">Line 1:</td>
<td colspan="2" class="diff-lineno">Line 1:</td>
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<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<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>Version 2 of the [[Monad Transformer Library]] introduced some</div></td>
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<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<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>[[Incompatibilities between MTL 1 and MTL 2|incompatibilites]]</div></td>
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<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<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>relative to version 1.</div></td>
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<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<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>This page provides instructions for upgrading code written for</div></td>
</tr>
<tr>
<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<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>MTL 1 to work with MTL 2.</div></td>
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<td colspan="2" class="diff-empty"> </td>
<td class="diff-marker">+</td>
<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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<td class="diff-marker"> </td>
<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>Many packages written for earlier versions of MTL build unchanged with version 2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with version 1 without some CPP directives. Here are the fixes for common messages:</div></td>
<td class="diff-marker"> </td>
<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>Many packages written for earlier versions of MTL build unchanged with version 2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with version 1 without some CPP directives. Here are the fixes for common messages:</div></td>
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</table>YitzGalehttps://wiki.haskell.org/index.php?title=Upgrading_from_MTL_1_to_MTL_2&diff=37464&oldid=prevYitzGale: Introductory sentence was taken out of context2010-11-16T11:37:29Z<p>Introductory sentence was taken out of context</p>
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<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">← Older revision</td>
<td colspan="2" style="background-color: #fff; color: #202122; text-align: center;">Revision as of 11:37, 16 November 2010</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><del class="diffchange diffchange-inline">Most</del> packages build unchanged with <del class="diffchange diffchange-inline">mtl-</del>2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with <del class="diffchange diffchange-inline">mtl-</del>1 without some CPP directives. Here are the fixes for common messages:</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">Many</ins> packages<ins class="diffchange diffchange-inline"> written for earlier versions of MTL</ins> build unchanged with <ins class="diffchange diffchange-inline">version </ins>2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with <ins class="diffchange diffchange-inline">version </ins>1 without some CPP directives. Here are the fixes for common messages:</div></td>
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</table>YitzGalehttps://wiki.haskell.org/index.php?title=Upgrading_from_MTL_1_to_MTL_2&diff=37463&oldid=prevYitzGale: Copied text from the Haskell Platfrom trac wiki page2010-11-16T11:34:42Z<p>Copied text from the Haskell Platfrom trac wiki page</p>
<p><b>New page</b></p><div>Most packages build unchanged with mtl-2. Most of the remainder require only small changes to upgrade to it, though that will usually render them incompatible with mtl-1 without some CPP directives. Here are the fixes for common messages:<br />
<br />
''Illegal instance declaration for Applicative (State a)''<br />
:This usually means that the instance you're defining is now defined in the [http://hackage.haskell.org/package/transformers transformers] package. You may wish to check it's the same, so you can just delete it.<br />
<br />
''Could not deduce (Functor m) from the context (Monad m)<br/>arising from a use of fmap''<br />
:This will be a situation where you're using a monad constructed with a transformer as a Functor. You can replace <hask>fmap</hask> with <hask>liftM</hask> and the code will build with old and new versions of mtl.<br />
<br />
''Could not deduce (Functor m) from the context (Monad m)<br/>arising from the superclasses of an instance declaration''<br />
:This will be another situation where you're using a monad constructed with a transformer as a Functor. You can replace the <hask>Monad m</hask> with <hask>Functor m</hask> to make it work with mtl-2, or add a <hask>Functor m</hask> constraint to make it work with old and new versions of mtl.<br />
<br />
''Not in scope: data constructor State''<br />
:The <hask>State</hask> type is now a type synonym. You could replace the <hask>State</hask> data constructor with the <hask>state</hask> function.<br />
<br />
''Not in scope: runState''<br />
:You probably imported <hask>State(..)</hask>, which won't work now that <hask>State</hask> is a type synonym. You need to import <hask>State</hask> and <hask>runState</hask>. (That will work with old versions of mtl too.)<br />
<br />
''Illegal instance declaration for Myclass (State Foo)''<br />
:If you have a matching instance for <hask>StateT</hask>, you can delete the instance for <hask>State</hask>. Otherwise you need to generalize your instance to <hask>StateT</hask>. If that's not possible, you may need to introduce a <hask>newtype</hask>.<br />
<br />
[[Category: Monad]]</div>YitzGale