wrong import for catch #88
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Should import Control.Exception , not System.IO.Error Signed-off-by: JadAbouHawili <[email protected]>
JadAbouHawili
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xogcox
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need Control.Exception for catch , System.IO.Error for IOError Signed-off-by: JadAbouHawili <[email protected]>
Signed-off-by: JadAbouHawili <[email protected]>
ulysses4ever
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Dec 17, 2025
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Hey @JadAbouHawili! We have just switched from HTML to Markdown. Could you please rebase (possibly re-do) you change on top of the current main branch?
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Dec 29, 2025
| <p>We did <span class="fixed">fileExists <- doesFileExist fileName</span> because <span class="fixed">doesFileExist</span> has a type of <span class="fixed">doesFileExist :: FilePath -> IO Bool</span>, which means that it returns an I/O action that has as its result a boolean value which tells us if the file exists. We can't just use <span class="fixed">doesFileExist</span> in an <i>if</i> expression directly.</p> | ||
| <p>Another solution here would be to use exceptions. It's perfectly acceptable to use them in this context. A file not existing is an exception that arises from I/O, so catching it in I/O is fine and dandy.</p> | ||
| <p>To deal with this by using exceptions, we're going to take advantage of the <span class="label function">catch</span> function from <span class="fixed">System.IO.Error</span>. Its type is <span class="fixed">catch :: IO a -> (IOError -> IO a) -> IO a</span>. It takes two parameters. The first one is an I/O action. For instance, it could be an I/O action that tries to open a file. The second one is the so-called handler. If the first I/O action passed to <span class="fixed">catch</span> throws an I/O exception, that exception gets passed to the handler, which then decides what to do. So the final result is an I/O action that will either act the same as the first parameter or it will do what the handler tells it if the first I/O action throws an exception.</p> | ||
| <p>To deal with this by using exceptions, we're going to take advantage of the <span class="label function">catch</span> function from <span class="fixed">Control.Exception</span>. Its type is <span class="fixed">catch :: IO a -> (IOError -> IO a) -> IO a</span>. It takes two parameters. The first one is an I/O action. For instance, it could be an I/O action that tries to open a file. The second one is the so-called handler. If the first I/O action passed to <span class="fixed">catch</span> throws an I/O exception, that exception gets passed to the handler, which then decides what to do. So the final result is an I/O action that will either act the same as the first parameter or it will do what the handler tells it if the first I/O action throws an exception.</p> |
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If we use Control.Exception.catch, the type needs to be changed to Exception e => IO a -> (e -> IO a) -> IO a. Alternatively, System.IO.Error.catchIOError could be used.
The Haskell report defines System.IO.Error.catch :: IO a -> (IOError -> IO a) -> IO a (which is where this issue comes from), which was then renamed to catchIOError in GHC (and MicroHs).
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catch is imported from Control.Exception , not System.IO.Error