Parse and implement class for permutation gates

[notmgsk]

Introduces a class DefPermutationGate which subclasses DefGate, so that gates can be defined using a permutation matrix: cnot = DefPermutationGate([0, 1, 3, 2]). Normal DefGate stuff works, like DefGate.num_args() and DefGate.get_constructor().

Also update the parser to accept permutations:

In [11]: p = Program("""DEFGATE CCNOT AS PERMUTATION:
    ...:     0, 1, 2, 3, 4, 5, 7, 6
    ...: CCNOT 3 2 1""")

In [12]: p._defined_gates
Out[12]: [<pyquil.quilbase.DefPermutationGate at 0x121a63c50>]

In [13]: p._defined_gates[0]
Out[13]: <pyquil.quilbase.DefPermutationGate at 0x121a63c50>

In [14]: p._defined_gates[0].get_constructor()
Out[14]: <function pyquil.quilbase.DefGate.get_constructor.<locals>.<lambda>(*qubits)>

In [15]: p._defined_gates[0].get_constructor()(0, 1, 2)
Out[15]: <Gate CCNOT 0 1 2>

In [16]: p._defined_gates[0].get_constructor()(0, 1)
Out[16]: <Gate CCNOT 0 1>

In [17]: p._defined_gates[0].num_args()
Out[17]: 3

[ecpeterson]

I'm a little surprised at In [16]. It doesn't throw a fit when you pass it too few qubit arguments?

[ecpeterson]

remarkably painless, lgtm

[notmgsk]

@ecpeterson DefGate does not throw an error, either. I can change that behaviour for both if you'd prefer.

pyQuil is a bit messy in this regard. Some places make sanity checks, others don't.

[ecpeterson]

I think I'd like that sanity check to be made when possible. I can't imagine anyone is relying on this bad behavior of generating illegal Quil. I suppose it doesn't have to be in this PR, though.

[ecpeterson]

Still looks fine to me.

Out of curiosity: when a user writes a Quil program with a bad permutation (like 0, 1, 1, 3), who eventually catches this error and what error do they signal?

[notmgsk]

Still looks fine to me.

Out of curiosity: when a user writes a Quil program with a bad permutation (like 0, 1, 1, 3), who eventually catches this error and what error do they signal?

After this is merged quil-lang/quilc#234, the above is caught at parse time in quilc.

rpcq._utils.RPCError: At line 1: Permutation gate entries do not represent a square matrix

[ecpeterson]

Cool. Still curious: what happens without this new quilc guard? Does tweedledum throw an error? Does it just give garbage results? (As an aside, I think the Quil spec promises only garbage-in-garbage-out with regards to non-unitary DEFGATEs, and we only deign to throw explicit errors as it suits us.)

[notmgsk]

@ecpeterson something unhelpful like

The value
  #C(#<DOUBLE-FLOAT quiet NaN> 1.5707963267948966d0)
is not of type
  DOUBLE-FLOAT

[karalekas]

As we discussed @notmgsk this change is not backwards compatible with older versions of quilc (<= 1.7.2 I believe), and so we should make the extra effort to warn (and dare I say prevent) people from using this feature with those versions of quilc.

[stylewarning]

one little change, otherwise i think this is Good 2 Go (TM).

The change requested above has been made