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Display type class members and let identifiers in errors - #3463

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LiamGoodacre:feature/instance-let-error-message
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Display type class members and let identifiers in errors#3463
LiamGoodacre wants to merge 1 commit into
purescript:masterfrom
LiamGoodacre:feature/instance-let-error-message

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@LiamGoodacre

@LiamGoodacre LiamGoodacre commented Dec 1, 2018

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Example code:

class Abc a where
  abc :: a -> a

instance abcInt :: Abc Int where
  abc a = let bac :: Int -> Int
              bac = 3 in bac a

Error diff:

    Could not match type
      Int
    with type
      Int -> Int

  while checking that type Int
    is at least as general as type Int -> Int
  while checking that expression 3
    has type Int -> Int
+ in value declaration bac
+ in value declaration abc
  in value declaration abcInt

I think this could be sufficient for the second half of #3390.

@LiamGoodacre LiamGoodacre mentioned this pull request Dec 1, 2018
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@LiamGoodacre

LiamGoodacre commented Dec 2, 2018

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Okay I broke something, this isn't quite ready yet.

@LiamGoodacre

LiamGoodacre commented Dec 2, 2018

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Feels a bit hacky too. Will re-think.

@hdgarrood

hdgarrood commented Mar 7, 2019

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It's perhaps worth noting that the --verbose-errors flag already gets you something somewhat similar (if a bit more, uh, verbose):

  Could not match type
       
    Int
       
  with type
              
    Int -> Int
              

while trying to match type Int
  with type Int -> Int
while checking that type Int
  is at least as general as type Int -> Int
while checking that expression 3
  has type Int -> Int
while checking that expression let      
                                 bac = 3
                               in bac a 
  has type Int
while checking that expression \a ->      
                                 let      
                                   bac = 3
                                 in bac a 
  has type Int -> Int
while checking that expression Abc { abc: \a ->      
                                            let      
                                              bac = 3
                                            in bac a 
                                   }                 
  has type { abc :: Int -> Int
           }                  
in value declaration abcInt

@hdgarrood

hdgarrood commented Mar 7, 2019 •
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I'm not sure how I feel about this approach actually. I think I'd prefer we leave this as is and instead improve error spans and implement printing actual code in errors (#1898) to address the issue of it being difficult to work out which bit of your code is causing the problem.

@hdgarrood

hdgarrood commented Apr 14, 2019

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Shall we close this (and #3390)?

@LiamGoodacre
LiamGoodacre deleted the feature/instance-let-error-message branch April 20, 2019 15:40
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@LiamGoodacre @hdgarrood