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Add a FromParam instance for String. #16

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Add a FromParam instance for String. #16

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4 changes: 4 additions & 0 deletions fn/src/Web/Fn.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -423,6 +423,10 @@ instance FromParam Text where
fromParam [x] = Right x
fromParam [] = Left ParamMissing
fromParam _ = Left ParamTooMany
instance {-# INCOHERENT #-} FromParam String where
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Can you tell me a little more about why "INCOHERENT" is needed here? I've never seen it before.

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Hi. The reason is that String is actually [Char] and so GHC complains that it is overlapped with the FromParam [a] instance below. (According to the doc, GHC will not take context (i.e. constrains before =>) into account when matching instances.)
One solution is to add "INCOHERENT" pragma according to this. I am not sure if there is a better solution.

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That makes sense. Let me think a little about whether this feature is worth that extra bit of type complexity. Regardless I appreciate you taking the time to make this PR

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No problem. Thank you very much!

fromParam [t] = Right (T.unpack t)
fromParam [] = Left ParamMissing
fromParam _ = Left ParamTooMany
instance FromParam Int where
fromParam [t] = case decimal t of
Left _ -> Left ParamUnparsable
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2 changes: 2 additions & 0 deletions fn/test/Spec.hs
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Expand Up @@ -170,6 +170,8 @@ main = hspec $ do
describe "parameter parsing" $
do it "should parse Text" $
fromParam ["hello"] `shouldBe` Right ("hello" :: Text)
it "should parse String" $
fromParam ["hello"] `shouldBe` Right ("hello" :: String)
it "should parse Int" $
do fromParam ["1"] `shouldBe` Right (1 :: Int)
fromParam ["2011"] `shouldBe` Right (2011 :: Int)
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