Unused variable warning affects compositionality when generating code

If I'm generating a program using template haskell, I might define a combinator to help me generate lambdas.

type Code a = Q (TExp a)

_lam :: (Code a -> Code b) -> Code (a -> b)
_lam f = [|| \a -> $$(f [|| a ||]) ||]

However, if I now pass a constant function into _lam, the generated code contains an unused variable a that I can do nothing about and desire not to do anything about.

c5 :: Code (a -> Int)
c5 = _lam (const [|| 5 ||])

However, GHC decides that it's wise to warn me about this when I splice it in.

> $$c5
F2.hs:6:5: warning: [-Wunused-matches] Defined but not used: ‘a’
  |
6 | q = $$c5
  |     ^^^

As Ryan will tell you, I'm against emitting warnings from generated code as it breaks the abstraction. The code that is generated is guaranteed to be type and scope correct but any aesthetic warning is irrelevant to the consumer.

I see it in precisely the same way as warning if we use a function that contains an unused variable in a library. Once the definition is exported from the library, its definition is opaque. The same principle should be applied to code generated by template haskell.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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mentioned in issue #14838

changed weight to 5

added Tbug Trac import labels

It would be easy to suppress all warnings in generated code. We already suppress some.

added TemplateHaskell + 1 deleted label

There's an unfortunate tension here, since other folks have requested //more// warnings from generated code (in #14838).

added typed TemplateHaskell label

added Pnormal label

As I have indicated in #14838, I think we want to suppress warnings for Generated code, but mark TH quotations (which concern #14838 and #17270 (closed)) as FromSource. That would solve both problems at the same time.

One more reason why we don't want warnings for Generated code: _lam from the OP might have been defined in a library that we have no control of. By changing the definition of _lam between library versions, users might experience warnings even though they didn't actually change anything in their code, also it's impossible for users to take measures to resolve those warnings.

For the record, marking quotations as FromSource wouldn't work, as I realised they are just data, which can be defined in a different module, by a different user and they don't even need to end up in a splice.

marked this issue as related to #17270 (closed)

mentioned in issue #14899 (closed)