[Haskell-cafe] Human-friendly compiler errors for GHC

Max Bolingbroke batterseapower at hotmail.com
Sat Jul 12 12:44:52 EDT 2008

I had some free time this afternoon so I put together an
(experimental) patch for GHC that implements helpful errors messages.
Have a look at this GHCi session to see what I mean:

$ stage2/ghc-inplace --interactive -fhelpful-errors
GHCi, version 6.9.20080711: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude> let foo = 10
Prelude> foa

    Not in scope: `foa'
    Maybe you meant one of:
      `not' -- Maybe the matching threshold could stand to be tweaked
a bit, e.g. scaled with identifier string length..
Prelude> let myIdentifier = 10
Prelude> myIdentfiier

    Not in scope: `myIdentfiier'
    Maybe you meant `myIdentifier'

The feature was inspired by the equivalent feature in the Boo
programming language (http://boo.codehaus.org/). I use the restricted
Damerau–Levenshtein distance to do the fuzzy match

What do you think about this feature? Would it be genuinely helpful or annoying?


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