Heuristics to improve error messages for badly referenced things

I propose the implementation of fuzzy-matching heuristics so we can get errors like this:

Prelude> fsa

<interactive>:1:0:
    Not in scope: `fsa'
    Maybe you meant `fst'
Prelude> import Data.Lits
Could not find module `Data.Lits':
  Use -v to see a list of the files searched for.
  Maybe you meant `Data.Bits' or `Data.List'
Prelude> 

And a heuristic such that when resolution of an unqualified name fails it can suggest a qualified alternative:

module Main where
import qualified Char

main = print (isSpace ' ')

$ stage2/ghc-inplace --make Test.hs
[1 of 1] Compiling Main             ( Test.hs, Test.o )

Test.hs:4:14:
    Not in scope: `isSpace'
    Maybe you meant `Char.isSpace'

I believe both of these represent an improvement to the compiler user experience.

If you wish to implement this feature in GHC, I have implemented, documented and tested it already and the patches are attached. It is under flag control: use -fno-helpful-errors to disable it. This is turned by default for the testsuite.

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Unknown
Architecture	Unknown