B as alias is not parsed

[quantumkoen]

This code from the qx simulator test suite is not parsed:

version 1.0 

# file   : full_adder.qc
# author : Nader Khammassi
# brief  : quantum full adder (carry-save addition)

# define 4 qubits
qubits 4

# rename qubits
map q[0],A 
map q[1],B
map q[2],C
map q[3],D


# init inputs to some values
.init
  x A 
  x C 

  display_binary  #  initial input value


# perform addition
.add

  toffoli B,C,D
  cnot    B,C 
  toffoli A,C,D
  cnot    A,C 

  display_binary #  result

.reverse_add

  cnot    A,C 
  toffoli A,C,D
  cnot    B,C 
  toffoli B,C,D

  display_binary #  result

# error_model depolarizing_channel, 0.1 

Reports:

[+] loading circuit from 'tests/circuits/full_adder.qc' ...
Error while parsing file tests/circuits/full_adder.qc: 
syntax error, unexpected BITHEAD, expecting NAME

[kel85uk]

@quantumkoen And A, C without the B works? I think it's because of the precedence of b being parsed as a special token for bit identifiers.

[quantumkoen]

@kel85uk hard to say what works exactly and what doesn't, as I'm not getting any line numbers with the syntax errors, but I suspect it's the B indeed because of the b[..] syntax.

[kel85uk]

Yeap, tested using the new develop branch with the line reporting. It does say line 12, which is where b is located.

Consider the code:

version 1.0 

# file   : full_adder.qc
# author : Nader Khammassi
# brief  : quantum full adder (carry-save addition)

# define 4 qubits
qubits 4

# rename qubits
map q[0],A 
map q[1],B1
map q[2],C
map q[3],D


# init inputs to some values
.init
  x A 
  x C 

  display_binary  #  initial input value


# perform addition
.add

  toffoli B1,C,D
  cnot    B1,C 
  toffoli A,C,D
  cnot    A,C 

  display_binary #  result

.reverse_add

  cnot    A,C 
  toffoli A,C,D
  cnot    B1,C 
  toffoli B1,C,D

  display_binary #  result

# error_model depolarizing_channel, 0.1 

which results in:

[kel85uk]

Question is, should b be supported as a string and not a special keyword, then if so, q would be open for such interpretation too. @quantumkoen @Nader-Khammassi @imranashraf @andreasppg

[kel85uk]

Closing due to no response, and the way I interpret it is that b and q should be a special keyword for cQASM the same way as special registers (e.g. ax) are reserved for classical assembly.

[quantumkoen]

Given that it is trivial to distinguish between the use of b as a signifier for adressing bits vs using it as an alias, and given that the cqasm 1.0 syntax nowhere says b cannot be used as an alias, I think it should be supported by the parser.

[kel85uk]

True, reopening this.

[jvanstraten]

This has been supported since #105, though due to a bug I stumbled on and fixed in #115 I think that particular mapping has only worked since then. Note that you wouldn't be able to refer to the measurement bits associated with the qubits once you redefine b though (unless you make another remap b first).