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<h3>
Marge Bot pushed to branch master
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<h4>
Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/045139f40089f288866c1c59c7379be82ecdaf34">045139f4</a></strong>
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<span>by Hécate</span>
<i>at 2020-04-09T23:10:44-04:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Add an example to liftIO and explain its purpose
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<h4>1 changed file:</h4>
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<a href="#991889f1959a811b406faf500d1a82d8fadc3935">
libraries/base/Control/Monad/IO/Class.hs
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<h4>Changes:</h4>
<li id="991889f1959a811b406faf500d1a82d8fadc3935">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/045139f40089f288866c1c59c7379be82ecdaf34#991889f1959a811b406faf500d1a82d8fadc3935"><strong>libraries/base/Control/Monad/IO/Class.hs</strong></a>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -30,8 +30,42 @@ module Control.Monad.IO.Class (</td>
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<pre style="margin: 0;"> <span id="LC30" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC31" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">class</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">Monad</span> <span class="n" style="color: #333;">m</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=></span> <span class="kt" style="color: #458; font-weight: 600;">MonadIO</span> <span class="n" style="color: #333;">m</span> <span class="kr" style="font-weight: 600;">where</span></span>
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<pre style="margin: 0;"> <span id="LC32" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- | Lift a computation from the 'IO' monad.</span></span>
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<pre style="margin: 0;">+<span id="LC33" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations</span></span>
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<pre style="margin: 0;">+<span id="LC34" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- (i.e. 'IO' is the base monad for the stack).</span></span>
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<pre style="margin: 0;">+<span id="LC35" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC36" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- === __Example__</span></span>
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<pre style="margin: 0;">+<span id="LC37" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC38" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC39" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > import Control.Monad.Trans.State -- from the "transformers" library</span></span>
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<pre style="margin: 0;">+<span id="LC40" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- ></span></span>
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<pre style="margin: 0;">+<span id="LC41" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > printState :: Show s => StateT s IO ()</span></span>
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<pre style="margin: 0;">+<span id="LC42" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > printState = do</span></span>
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<pre style="margin: 0;">+<span id="LC43" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > state <- get</span></span>
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<pre style="margin: 0;">+<span id="LC44" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > liftIO $ print state</span></span>
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<pre style="margin: 0;">+<span id="LC45" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC46" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC47" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Had we omitted @'liftIO'@, we would have ended up with this error:</span></span>
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<pre style="margin: 0;">+<span id="LC48" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC49" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > • Couldn't match type ‘IO’ with ‘StateT s IO’</span></span>
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<pre style="margin: 0;">+<span id="LC50" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > Expected type: StateT s IO ()</span></span>
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<pre style="margin: 0;">+<span id="LC51" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > Actual type: IO ()</span></span>
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<pre style="margin: 0;">+<span id="LC52" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC53" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- The important part here is the mismatch between @StateT s IO ()@ and @'IO' ()@.</span></span>
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<pre style="margin: 0;">+<span id="LC54" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC55" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Luckily, we know of a function that takes an @'IO' a@ and returns an @(m a)@: @'liftIO'@,</span></span>
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<pre style="margin: 0;">+<span id="LC56" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- enabling us to run the program and see the expected results:</span></span>
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<pre style="margin: 0;">+<span id="LC57" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC58" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- @</span></span>
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<pre style="margin: 0;">+<span id="LC59" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > evalStateT printState "hello"</span></span>
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<pre style="margin: 0;">+<span id="LC60" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- "hello"</span></span>
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<pre style="margin: 0;">+<span id="LC61" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">+<span id="LC62" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- > evalStateT printState 3</span></span>
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<pre style="margin: 0;">+<span id="LC63" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- 3</span></span>
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<pre style="margin: 0;">+<span id="LC64" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- @</span></span>
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<pre style="margin: 0;">+<span id="LC65" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;"> <span id="LC66" class="line" lang="haskell"> <span class="n" style="color: #333;">liftIO</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">IO</span> <span class="n" style="color: #333;">a</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">m</span> <span class="n" style="color: #333;">a</span></span>
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<pre style="margin: 0;"> <span id="LC67" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC68" class="line" lang="haskell"><span class="cd">-- | @since 4.9.0.0</span></span>
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<pre style="margin: 0;"> <span id="LC69" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">instance</span> <span class="kt" style="color: #458; font-weight: 600;">MonadIO</span> <span class="kt" style="color: #458; font-weight: 600;">IO</span> <span class="kr" style="font-weight: 600;">where</span></span>
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<pre style="margin: 0;"> <span id="LC70" class="line" lang="haskell"> <span class="n" style="color: #333;">liftIO</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">id</span></span>
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<pre style="margin: 0;">+<span id="LC71" class="line" lang="haskell"></span></pre>
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