Bad error message in GHCi (#675) · Issues · Glasgow Haskell Compiler / GHC

Bad error message in GHCi

import Test.QuickCheck

prop_eq_reflexive x = x == x

is loaded into GHCi the following can be observed:

*BadErrorMessage> quickCheck prop_eq_reflexive

Top level:
    No instance for (Show (IO ()))
      arising from use of `print' at Top level
    Probable fix: add an instance declaration for (Show (IO ()))
    In a 'do' expression: print it

Hugs gives the more understandable error message

BadErrorMessage> quickCheck prop_eq_reflexive
ERROR - Unresolved overloading
*** Type       : (Eq a, Show a, Arbitrary a) => IO ()
*** Expression : quickCheck prop_eq_reflexive

:t in GHCi gives an even more understandable error message:

*BadErrorMessage> :t quickCheck prop_eq_reflexive

<interactive>:1:0:
    Ambiguous type variable `a' in the constraints:
      `Arbitrary a' arising from use of `quickCheck' at <interactive>:1:0-9
      `Eq a' arising from use of `prop_eq_reflexive' at <interactive>:1:11-27
      `Show a' arising from use of `quickCheck' at <interactive>:1:0-9
    Probable fix: add a type signature that fixes these type variable(s)

(In the introductory FP courses at Chalmers we use Quick``Check from the start, so this error message bites students in the first week.)

Trac metadata

Trac field	Value
Version	6.4.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown

```
import Test.QuickCheck

prop_eq_reflexive x = x == x
```

is loaded into GHCi the following can be observed:

```
*BadErrorMessage> quickCheck prop_eq_reflexive

Top level:
    No instance for (Show (IO ()))
      arising from use of `print' at Top level
    Probable fix: add an instance declaration for (Show (IO ()))
    In a 'do' expression: print it
```

Hugs gives the more understandable error message

```
BadErrorMessage> quickCheck prop_eq_reflexive
ERROR - Unresolved overloading
*** Type       : (Eq a, Show a, Arbitrary a) => IO ()
*** Expression : quickCheck prop_eq_reflexive
```

`:t` in GHCi gives an even more understandable error message:

```
*BadErrorMessage> :t quickCheck prop_eq_reflexive

<interactive>:1:0:
    Ambiguous type variable `a' in the constraints:
      `Arbitrary a' arising from use of `quickCheck' at <interactive>:1:0-9
      `Eq a' arising from use of `prop_eq_reflexive' at <interactive>:1:11-27
      `Show a' arising from use of `quickCheck' at <interactive>:1:0-9
    Probable fix: add a type signature that fixes these type variable(s)
```

(In the introductory FP courses at Chalmers we use Quick\`\`Check from the start, so this error message bites students in the first week.)

<details><summary>Trac metadata</summary>

| Trac field             | Value          |
| ---------------------- | -------------- |
| Version                | 6.4.1          |
| Type                   | FeatureRequest |
| TypeOfFailure          | OtherFailure   |
| Priority               | normal         |
| Resolution             | Unresolved     |
| Component              | GHCi           |
| Test case              |                |
| Differential revisions |                |
| BlockedBy              |                |
| Related                |                |
| Blocking               |                |
| CC                     |                |
| Operating system       | Unknown        |
| Architecture           | Unknown        |

</details>