Proposal to deprecate and then drop fromJust

[Simon Peyton Jones]

We are working on detecting partiality in pattern matching right now

S

|  -----Original Message-----
|  From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of
|  Richard Eisenberg
|  Sent: 24 February 2015 15:02
|  To: Herbert Valerio Riedel
|  Cc: libraries at haskell.org; Joachim Breitner
|  Subject: Re: Proposal to deprecate and then drop fromJust
|  
|  I've been wanting to get warnings on partial functions (and partial
|  record accessors, while we're at it!) for some time.
|  
|  And, I think it would be quite easy to have GHC infer partiality. As
|  far as I can see, partiality can come from only two places: the use of
|  a partial function, and an incomplete pattern match. (I'm ignoring
|  nontermination here, which I think is best considered separately.)
|  Before getting this into GHC, though, we'd need to make sure the
|  incomplete-pattern-match checker is up for it (see #3927), but that
|  work is being done right now. And we'd definitely need a way for users
|  to declare "This function is total! Really!"
|  
|  I like the idea of user-specified warning categories, but I think
|  partiality is easy enough for GHC to infer that it should be built in.
|  
|  +1 on partiality warnings, preferably inferred
|  +0.8 on user-specified warning classes (but inferred partiality is my
|  +preference)
|  
|  -0.9 on just attaching a one-off warning to fromJust (we can do so
|  much better)
|  -1 on removing fromJust (I use it myself [don't hate me for it], and
|  the GHC codebase uses `expectJust`, which takes a string for better
|  error messages, 54 times by my count)
|  
|  Richard
|  
|  On Feb 24, 2015, at 7:28 AM, Herbert Valerio Riedel <hvr at gnu.org>
|  wrote:
|  
|  > On 2015-02-24 at 12:32:14 +0100, Michael Snoyman wrote:
|  >> We could also automatically detect basic partiality, like
|  incomplete
|  >> pattern matches, and add partiality warnings. Depending on how much
|  >> work that is, it may be easier than trying to mark up all of base
|  >> correctly.
|  >
|  > You'd still need a way to greenlight false positives. I.e. cases
|  where
|  > the code *seems* to have unhandled pattern matches which can't
|  happen,
|  > but which GHC can't prove them not to be unhandled...
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