Provide information of the instance environment (#1835) · Issues · Glasgow Haskell Compiler / GHC

Provide information of the instance environment

This feature request was brought up in [1].

Right now TH doesn't have access to the instance environment of the typechecker. Thus, it's impossible to check whether a class instance exists for certain type, making impossible to guarantee the generation of correct code in some cases.

For example, that happens when, for a justified reason, the function to be spliced takes a variable Name as argument and not a value. There is no way to force a class constraint on names nor a way to check whether the variable type (obtained with reify) is an instance of a certain class. If the generated code makes use of any class function, its possible to generate incorrect code.

I suggest adding a function similar to this one:

isClassInstance :: Name   -- ^ Class name
                -> Type   -- ^ Type to check
                -> Q Bool

Another option would be extending ClassI to provide the list of instances of a class and code 'isClassInstance' using that information.

[1] http://www.haskell.org/pipermail/template-haskell/2007-November/000645.html

Trac metadata

Trac field	Value
Version	6.8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	alfonso.acosta@gmail.com
Operating system	Unknown
Architecture	Unknown

This feature request was brought up in \[1\].

For example, that happens when, for a justified reason, the function to be spliced takes a variable Name as argument and not a value. There is no way to force a class constraint on names nor a way to check whether the variable type (obtained with reify) is an instance of a certain class. If the generated code makes use of any
class function, its possible to generate incorrect code.

I suggest adding a function similar to this one:

```
isClassInstance :: Name   -- ^ Class name
                -> Type   -- ^ Type to check
                -> Q Bool
```

Another option would be extending ClassI to provide the list of instances of a class and code 'isClassInstance' using that information.

\[1\] http://www.haskell.org/pipermail/template-haskell/2007-November/000645.html

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

| Trac field             | Value                    |
| ---------------------- | ------------------------ |
| Version                | 6.8.1                    |
| Type                   | FeatureRequest           |
| TypeOfFailure          | OtherFailure             |
| Priority               | normal                   |
| Resolution             | Unresolved               |
| Component              | Template Haskell         |
| Test case              |                          |
| Differential revisions |                          |
| BlockedBy              |                          |
| Related                |                          |
| Blocking               |                          |
| CC                     | alfonso.acosta@gmail.com |
| Operating system       | Unknown                  |
| Architecture           | Unknown                  |

</details>