Template haskell API makes inconsistent use of [Q Dec], Q [Dec] and Q Dec
Template haskell declaration quotations produce Q [Dec]. However, all of the functions for consuming lists of declarations in Language.Haskell.TH accept only [Q Dec]. This results in some pretty awkward constructs, such as:
instanceD ctxt inst =<< (fmap.fmap) return [d|
foo a = a * 42
|][Note also that |] must be indented in the above example (otherwise GHC gives an unhelpful error).]
Equally problematically, the functions which /create/ declarations all produce Q Dec, but declaration splices want Q [Dec], resulting in the need to write ugly code or drop into a do-block.
My modest proposal to fix all this is to have only a single type used for declarations within the world of TH. The obvious most-general choice for this type is Q [Dec]. So:[[br]]
- Change all [DecQ] arguments to Q [Dec][[br]]
- Change all DecQ return types to Q [Dec][[br]]
- Add "<++> = liftM2 (++)" for easy combining of Q [Dec] to Language.Haskell.TH.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 6.12.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Template Haskell |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
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