Where head is used (in this case on csMainLocals), protected it with a condition so that it gives a more useful error message than simply head: empty list

TLP.hs

@@ -39,6 +39,7 @@ data TLPChannel = TLPIn | TLPOut | TLPError
 tlpInterface :: (CSM m, Die m) => m (A.Name, [TLPChannel])
 tlpInterface
     =  do ps <- get
+          when (null $ csMainLocals ps) (die "No main process found")
           let mainName = snd $ head $ csMainLocals ps
           st <- specTypeOfName mainName
           formals <- case st of

Unnest.hs

@@ -29,6 +29,7 @@ import CompState
 import EvalConstants
 import Metadata
 import Pass
+import Errors
 import Types
 
 unnest :: A.Process -> PassM A.Process
@@ -111,6 +112,7 @@ removeFreeNames = doGeneric `extM` doSpecification `extM` doProcess
              -- we know it's not going to be moved by removeNesting, so anything
              -- that it had in scope originally will still be in scope.
              ps <- get
+             when (null $ csMainLocals ps) (die "No main process found")
              let isTLP = (snd $ head $ csMainLocals ps) == n
 
              -- Figure out the free names.