[GHC] #960: Lexical call site string

[GHC]

#960: Lexical call site string
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 Reporter:  paul at cogito.org.uk  |          Owner:         
     Type:  feature request     |         Status:  new    
 Priority:  normal              |      Milestone:         
Component:  Compiler            |        Version:  6.6    
 Severity:  normal              |     Resolution:         
 Keywords:                      |     Difficulty:  Unknown
 Testcase:                      |   Architecture:  Unknown
       Os:  Unknown             |  
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Comment (by pauljohnson):

 My first thought was that the head function should include something like

    head [] = error $ "head: empty string at " ++ callSite

 So then if my file Foo.hs at line 35 says "head []" at run-time I get an
 error message saying

 "head: empty string at Foo.hs line 35"

 However this is not referentially transparent: "callSite" is not a
 constant, and its probably not useful for it to be valid outside
 exceptions.  So instead how about

    callSite :: Exception -> String

 returns the file and line where the function that triggered the exception
 was called from.  Then
 the default exception handler can print the exception string followed by
 the output of callSite.

 As Neil Mitchell pointed out in an email, Hat can answer the question of
 where the erroneous function was called from.  But unless you are already
 using Hat its a lot of trouble to go to just to locate a simple error.

 Paul.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/960>
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