NotCPP.ScopeLookup

This module exports scopeLookup, which will find a variable or value constructor for you and present it for your use. E.g. at some point in the history of the acid-state package, openAcidState was renamed openLocalState; for compatibility with both, you could use:

openState :: IO (AcidState st)
openState = case $(scopeLookup "openLocalState") of
  Just open -> open defaultState
  Nothing -> case $(scopeLookup "openAcidState") of
    Just open -> open defaultState
    Nothing -> error
      "openState: runtime name resolution has its drawbacks :/"

Or, for this specific case, you can use scopeLookups:

openState :: IO (AcidState st)
openState = open defaultState
 where
  open = $(scopeLookups ["openLocalState","openAcidState"])

Now if neither of the names are found then TH will throw a compile-time error.

Documentation

scopeLookup :: String -> Q Exp

Produces a spliceable expression which expands to Just val if the given string refers to a value val in scope, or Nothing otherwise.

scopeLookup = fmap liftMaybe . scopeLookup'

scopeLookups :: [String] -> Q Exp

Finds the first string in the list that names a value, and produces a spliceable expression of that value, or reports a compile error if it fails.

scopeLookup' :: String -> Q (Maybe Exp)

Produces Just x if the given string names the value x, or Nothing otherwise.

liftMaybe :: Maybe Exp -> Exp

Turns Nothing into an expression representing Nothing, and Just x into an expression representing Just applied to the expression in x.

recoverMaybe :: Q a -> Q (Maybe a)

Turns a possibly-failing Q action into one returning a Maybe value.

maybeReify :: Name -> Q (Maybe Info)

A useful variant of reify that returns Nothing instead of halting compilation when an error occurs (e.g. because the given name was not in scope).

infoToExp :: Info -> Maybe Exp

Returns Just (VarE n) if the info relates to a value called n, or Nothing if it relates to a different sort of thing.