Missing case definition in Miranda

Question

I'm getting a missing case definition when I'm calling this

check c (n:nx) state (l:ls,r:rs)
=true,if((isprefix state c)&(r=n))
=false, otherwise

I've checked this and it works on its own no matter what I send it.

This is where I'm calling it from (warning: it's a bit badly written for now):

readword input state tape
=output tape, if (((haltcheck sWord sNum state tape)=true)&(isprefix " halt" rline))
=doinst rline state tape , if ((check sWord sNum state tape)=true)
=readword tail state tape, otherwise
  where
  theline = dropwhile notit input
  start = dropwhile  isspace theline
  sWord = takewhile letter start
  ends = dropwhile notspace start 
  distart = dropwhile isspace ends
  sNum = takewhile notspace distart
  tail = dropwhile notspace distart
  rline = takewhile notit tail

Answer 1

Missing case definition means that you're pattern matching and you don't cover all cases. This happens twice in the definition of your check function: You're matching the second parameter with the pattern n:nx, but not against the pattern [] (so you're not covering the case that the second argument may be the empty list). Similarly you're matching the fourth argument against (l:ls, r:rs), not accounting for the possibility that either of the pair's elements may be the empty list.

So what's happening to cause the error is that when you're calling check from readword either sNum is empty or one of the lists in tape is empty.