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Warn about possible exceptions during file decoding #1022

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Lysxia merged 1 commit into from
Jul 2, 2026
+8 −1
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Warn about possible exceptions during file decoding #1022

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9 changes: 8 additions & 1 deletion src/Data/Aeson.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -199,6 +199,8 @@ encodeFile fp = L.writeFile fp . encode
--
-- This function parses immediately, but defers conversion. See
-- 'json' for details.
--
-- Throws an 'Exception' when the file is missing.

@phadej phadej Jul 7, 2026

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This is misleading. decodeFileStrict (and others) throw all the errors B.readFile may throw, not only missing file. More correctly it should say "Doesn't modify exceptions thrown by Data.ByteString.readFile used to read the file". FWIW, readFile doesn't document any exception behavior either.

*Data.Aeson> decodeFileStrict "foo" :: IO (Maybe Int)
*** Exception: foo: withBinaryFile: permission denied (Permission denied)

@Lysxia Lysxia Jul 7, 2026

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It's a bit strong to call this "misleading", but feel free to make it more precise.

decodeFileStrict :: (FromJSON a) => FilePath -> IO (Maybe a)
decodeFileStrict = fmap decodeStrict . B.readFile

Expand Down Expand Up @@ -226,12 +228,15 @@ decodeStrict' = decodeStrict
-- If this fails due to incomplete or invalid input, 'Nothing' is
-- returned.
--
-- Since @2.2.0.0@ an alias for 'decodeFileStrict'.
-- Throws an 'Exception' when the file is missing.
--
-- Since @2.2.0.0@ an alias for 'decodeFileStrict'.
decodeFileStrict' :: (FromJSON a) => FilePath -> IO (Maybe a)
decodeFileStrict' = decodeFileStrict

-- | Like 'decodeFileStrict' but returns an error message when decoding fails.
--
-- Throws an 'Exception' when the file is missing.
eitherDecodeFileStrict :: (FromJSON a) => FilePath -> IO (Either String a)
eitherDecodeFileStrict =
fmap eitherDecodeStrict . B.readFile
Expand All @@ -253,6 +258,8 @@ eitherDecodeStrict' = eitherDecodeStrict

-- | Like 'decodeFileStrict'' but returns an error message when decoding fails.
--
-- Throws an 'Exception' when the file is missing.
--
-- Since @2.2.0.0@ an alias for 'eitherDecodeFileStrict'.
eitherDecodeFileStrict' :: (FromJSON a) => FilePath -> IO (Either String a)
eitherDecodeFileStrict' = eitherDecodeFileStrict
Expand Down
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