Implicit parameters can only be bound using simple let and where declarations. In this example, one would expect ?t to be bound to the result of getCurrentTime, however the program is rejected with a syntax error:

{-# LANGUAGE ImplicitParams #-}

import Data.Time.Clock

printTime :: (?t :: UTCTime) => IO ()
printTime = putStrLn $ show ?t

main = do
  ?t <- getCurrentTime
  printTime

Instead, one must first bind to a regular variable and then bind the implicit parameter to it:

{-# LANGUAGE ImplicitParams #-}

import Data.Time.Clock

printTime :: (?t :: UTCTime) => IO ()
printTime = putStrLn $ show ?t

main = do
  t <- getCurrentTime
  let ?t = t
  printTime

In general, it seems like any pattern binding involving implicit parameters could be rewritten as a pattern binding to ordinary variables, followed by a binding of the implicit parameters to the ordinary variables. So you could bind implicit parameters buried in variables, tuples, record fields, etc.:

Sugared:

{-# LANGUAGE ImplicitParams #-}

data Example = Example { a :: Int, b :: Int, c :: Int }

f :: (?a :: Int, ?b :: Int, ?c :: Int) => Int
f = ?a + ?b + ?c

main = do
  let x = Example 1 2 3
  let (Example { a = ?a, b = ?b, c = ?c }) = x
  putStrLn $ show f

Desugared:

{-# LANGUAGE ImplicitParams #-}

data Example = Example { a :: Int, b :: Int, c :: Int }

f :: (?a :: Int, ?b :: Int, ?c :: Int) => Int
f = ?a + ?b + ?c

main = do
  let x = Example 1 2 3
  let (Example { a = a, b = b, c = c }) = x
  let ?a = a
      ?b = b
      ?c = c
  putStrLn $ show f