#7222: The text "Possible fix: add an instance declaration for ..." is redundant
and not usually helpful
------------------------------+---------------------------------------------
Reporter: maltem | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.2 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Other | Testcase:
Blockedby: | Blocking:
Related: |
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The current state of affairs: Given a typical type error, for example
{{{
"a" + "b"
}}}
we get
{{{
No instance for (Num [Char])
arising from a use of `+'
Possible fix: add an instance declaration for (Num [Char])
In the expression: "a" + "b"
In an equation for `it': it = "a" + "b"
}}}
I'm concerned with the third line here:
1) It is redundant, for it just repeats information from the first line.
In my experience, the redundancy sometimes hinders usability, namely, when
the types get very long, visually burying the problematic expression.
2) Furthermore it is often misleading: Everyday type errors stem from
incorrect usage of a library, not from missing bits of a library. The line
tends to be confusing especially for new users: the only ones who might
have profited from the redundancy.
3) To expand a bit on (1): An imported bit of information in the type
error is the fact that the type mismatch lies in the application of (+) to
"a". The user gains this information by combination of lines 2 and 4. It
is incovenient that line 3 visually seperates those lines.
I propose that the problematic line be simply removed.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7222>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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