Understanding bind in template haskell

[Yotam Ohad]

Hi,
In the function DsMeta.repE there is a format being repeated:
For example in:

repE e@(ExplicitTuple _ es boxed)
  | not (all tupArgPresent es) = notHandled "Tuple sections" (ppr e)
  | isBoxed boxed = do { xs <- repLEs [e | (dL->L _ (Present _ e)) <- es]
                       ; repTup xs }
  | otherwise     = do { xs <- repLEs [e | (dL->L _ (Present _ e)) <- es]
                       ; repUnboxedTup xs }

There is  `(dL->L _ (Present _ e)) <- es`. I don't understand how this type
checks correctly.
If I'll try to do

 data Foo a = Foo a
runQ [| Foo b <- Foo 1 |]

in ghciI get an error. What is the difference?

Yotam
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