Re: [GHC] #14397: For type error involving inferred types, show source of the type

28 Oct 2017

      #14397: For type error involving inferred types, show source of the type
-------------------------------------+-------------------------------------
        Reporter:  max630            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Old description:
...
Consider the code:
{{{#!hs
module TestTypes where
a = 42
b = a ++ "hello"
}}}
The type Error is (ghc-8.2.1):
{{{
TestTypes.hs:3:5: error:
    • No instance for (Num [Char]) arising from the literal ‘42’
    • In the expression: 42
      In an equation for ‘a’: a = 42
  |
3 | a = 42
  |     ^^
}}}
Actually, the real mistake is in line 3, where `a` is inferred as of type
`[Char]`, while it was not meant to. But that location is not reported to
the error. As you can guess, in real code location where the unexpected
type is inferred for a binding is not that obvious to find.
Suggestion: track locations where bindings' types are inferred, and in
case of type error involvin the bindings, add them to the "Relevant
bindings" section, or otherwise show it.
New description:

 Consider the code:

 {{{#!hs
 module TestTypes where

 a = 42

 b = a ++ "hello"
 }}}

 The type Error is (ghc-8.2.1):

 {{{
 TestTypes.hs:3:5: error:
     • No instance for (Num [Char]) arising from the literal ‘42’
     • In the expression: 42
       In an equation for ‘a’: a = 42
   |
 3 | a = 42
   |     ^^
 }}}

 Actually, the real mistake is in line 5, where `a` is inferred as of type
 `[Char]`, while it was not meant to. But that location is not reported to
 the error. As you can guess, in real code location where the unexpected
 type is inferred for a binding is not that obvious to find.

 Suggestion: track locations where bindings' types are inferred, and in
 case of type error involvin the bindings, add them to the "Relevant
 bindings" section, or otherwise show it.

--

Comment (by max630):

 Fix the line reference in description

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14397#comment:3
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