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:                    |
-------------------------------------+-------------------------------------
Description changed by max630:

Old description:
...
Consider the code:
{{{#!hs
a = 42
b = a . "hello"
}}}
The type Error is (ghc-8.2.1):
{{{
TestTypes.hs:3:5: error:
    • Couldn't match expected type ‘b0 -> c’ with actual type ‘Integer’
    • In the first argument of ‘(.)’, namely ‘a’
      In the expression: a . "hello"
      In an equation for ‘b’: b = a . "hello"
    • Relevant bindings include b :: a -> c (bound at TestTypes.hs:3:1)
  |
3 | b = a . "hello"
  |     ^
TestTypes.hs:3:9: error:
    • Couldn't match expected type ‘a -> b0’ with actual type ‘[Char]’
    • In the second argument of ‘(.)’, namely ‘"hello"’
      In the expression: a . "hello"
      In an equation for ‘b’: b = a . "hello"
    • Relevant bindings include b :: a -> c (bound at TestTypes.hs:3:1)
  |
3 | b = a . "hello"
  |         ^^^^^^^
}}}
Actually, the real mistake may be in line 1, where `a` is inferred as of
type `Integer`, while it probably 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 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.

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14397#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler