[Regression] Only one clause allowed in (explicitly bidirectional) pattern synonyms
Regression. This worked in 7.10.2:
{-# LANGUAGE PatternSynonyms, ViewPatterns #-}
pattern A :: Int -> String
pattern A n <- (read -> n) where
A 0 = "hi"
A 1 = "bye"
Removing the final clause works in GHC head but given the same code it claims the clause is empty:
% ghci -ignore-dot-ghci /tmp/tmp.t0h0pMgwWb.hs
GHCi, version 8.1.20160105: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( /tmp/tmp.t0h0pMgwWb.hs, interpreted )
/tmp/tmp.t0h0pMgwWb.hs:4:9: error:
pattern synonym 'where' clause cannot be empty
In the pattern synonym declaration for: A
Failed, modules loaded: none.
Prelude>
The where clause is certainly not empty — ironically seems to be caused by my very own #10426 (closed) (D1665) :--) hoist by my own ticket as we say:
; when (length matches /= 1) (wrongNumberErr loc)
Personally a trailing where
is quite alright and handy when quickly checking if a declaration is otherwise OK. It works for data/newtype declarations as well as type classes. A workaround is to pattern match in other ways:
pattern A n <- ... where
A = \case
0 -> "hi"
1 -> "bye"