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 BoolAnother 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 |