- : 'then' exp { LL $ \leftStmts -> (mkTransformStmt (reverse leftStmts) $2) }
- | 'then' exp 'by' exp { LL $ \leftStmts -> (mkTransformByStmt (reverse leftStmts) $2 $4) }
- | 'then' 'group' 'by' exp { LL $ \leftStmts -> (mkGroupByStmt (reverse leftStmts) $4) }
- | 'then' 'group' 'using' exp { LL $ \leftStmts -> (mkGroupUsingStmt (reverse leftStmts) $4) }
- | 'then' 'group' 'by' exp 'using' exp { LL $ \leftStmts -> (mkGroupByUsingStmt (reverse leftStmts) $4 $6) }
+ -- Function is applied to a list of stmts *in order*
+ : 'then' exp { LL $ \leftStmts -> (mkTransformStmt leftStmts $2) }
+ -- >>>
+ | 'then' exp 'by' exp { LL $ \leftStmts -> (mkTransformByStmt leftStmts $2 $4) }
+ | 'then' 'group' 'by' exp { LL $ \leftStmts -> (mkGroupByStmt leftStmts $4) }
+ -- <<<
+ -- These two productions deliberately have a shift-reduce conflict. I have made 'group' into a special_id,
+ -- which means you can enable TransformListComp while still using Data.List.group. However, this makes the two
+ -- productions ambiguous. I've set things up so that Happy chooses to resolve the conflict in that case by
+ -- choosing the "group by" variant, which is what we want.
+ --
+ -- This is rather dubious: the user might be confused as to how to parse this statement. However, it is a good
+ -- practical choice. NB: Data.List.group :: [a] -> [[a]], so using the first production would not even type check
+ -- if /that/ is the group function we conflict with.
+ | 'then' 'group' 'using' exp { LL $ \leftStmts -> (mkGroupUsingStmt leftStmts $4) }
+ | 'then' 'group' 'by' exp 'using' exp { LL $ \leftStmts -> (mkGroupByUsingStmt leftStmts $4 $6) }