Somehow detect splicing in ghci

I'm no TH expert but currently it seems you need a hack (adding data X ; or pure []) to do the following

ghci> $(reify ''() >>= runIO.print >> return [])

<interactive>:210:3: error:
    • Couldn't match type ‘[t0]’ with ‘Exp’
      Expected type: ExpQ
        Actual type: Q [t0]
    • In the expression: reify ''() >>= runIO . print >> return []
      In the untyped splice: $(reify ''() >>= runIO . print >> return [])

>>> data X; $(reify ''() >>= runIO.print >> return [])
TyConI (DataD [] GHC.Tuple.() [] Nothing [NormalC GHC.Tuple.() []] [])

>>> pure []; $(reify ''() >>= runIO.print >> return [])
TyConI (DataD [] GHC.Tuple.() [] Nothing [NormalC GHC.Tuple.() []] [])

Same with makeLenses (discussed here, there, hither):

>>> data A = B { _int :: Int }
>>> makeLenses ''A

<interactive>:209:1: error:
    • No instance for (Show DecsQ) arising from a use of ‘print’
    • In a stmt of an interactive GHCi command: print it

the following two work

>>> data A = B { _int :: Int }
>>> pure []; makeLenses ''A

>>> view int (B 42)
42

>>> data A = B { _int :: Int }; makeLenses ''A

>>> view int (B 42)
42

mentioned in issue #7331

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GHCi sees splices as expression splices. You want a declaration splice. The truth is that either one is reasonable. But perhaps GHCi can detect what you mean by the types and Do The Right Thing. Just like it already does to detect when you want an IO action.

Trac metadata

Trac field	Value
Component	Compiler → Template Haskell

Replying to [ticket:12410#comment:122563 goldfire]:

But perhaps GHCi can detect what you mean by the types and Do The Right Thing. Just like it already does to detect when you want an IO action.

Yes I had something like that in mind, detect if the type is DecsQ

---

Side note:

-- WORKS

>>> pure @Q @[Dec] []; $(reify ''() >>= runIO.print >> return [])
TyConI (DataD [] GHC.Tuple.() [] Nothing [NormalC GHC.Tuple.() []] [])

-- WORKS

>>> (pure [] :: DecsQ); $(reify ''() >>= runIO.print >> return [])
TyConI (DataD [] GHC.Tuple.() [] Nothing [NormalC GHC.Tuple.() []] [])

-- DOESN'T

>>> pure [] :: DecsQ; $(reify ''() >>= runIO.print >> return [])

<interactive>:374:1: error:
    Invalid type signature: pure [] :: ...
    Should be of form <variable> :: <type>

Another alternative to get ghci to do declaration splices: { $(makeLenses ''A) }. Eric Mertens also suggests doing $(makeLenses ''A);

I like the suggestion in ticket:12410#comment:156138. It's short, and fairly intuitive. Perhaps we should just enshrine this in the users' guide and call this bug fixed?

What's wrong with ticket:12410#comment:122563? The GHCi prompt already is flexible over the type of the thing you write...

The GHCi prompt is only flexible because it has syntactic cues that it uses to distinguish between expressions, declarations, imports, etc. With Template Haskell splices, you have no such syntactic cues. The only way I could envision ticket:12410#comment:122563 ever being feasible is if we hacked the typechecker to treat Q Exp and Q [Dec]-returning splices differently, which is a lot of work for questionable benefit.

ticket:12410#comment:156153 has the advantage that it's simple, works today, and is consistent with how other expressions and declarations are treated in GHCi (for instance, 3; won't parse in GHCi's prompt). In light of this, I'm inclined to favor the simpler solution.

OK -- I won't fight. :)

See also #7331.

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