One example of such sets: Define Q as the set of turing machines, which halt on empty input. Define P as the set of turing machines, which halt on every input. Clearly P ⊂ Q and P is undecidable and not semidecidable, but Q is undecidable and semidecidable....

parsing,unification,idris,decidable

The error message is correct: you provided a value of type Type, but you need a value of type p '0'. You are also correct that p is of type Char -> Type, and therefore that p '0' is of type Type. However, p '0' is not of type p...

I think you are right. To the best of my knowledge, you can't even correctly state what it means for two streams to be equal, since it would imply that you can inspect them in finite time, but they are infinite terms. What you could do, is state that any...

computation-theory,turing-machines,np-complete,np,decidable

This statement is false. Since 3SAT is NP-complete, every problem in NP polynomial-time reduces to 3SAT, so if you choose any language in NP, then it will polynomial-time reduce to 3SAT. In particular, if you choose the empty language ∅, which is known not to be NP-complete, then ∅ ∈...