def from_json(cls, file):
with open(file, 'r') as f:
spec = json.load(f)
terms = spec.get('T')
nonterms = spec.get('N')
prods = spec.get('P')
axiom = spec.get('S')
if None in [terms, nonterms, prods, axiom]:
raise ValueError('Incompatible specification')
terms = set(map(Symbol.new, terms))
nonterms = set(map(Symbol.new, nonterms))
prods = list(map(Production.from_string, prods))
axiom = Symbol.new(axiom)
return cls(terms, nonterms, prods, axiom)
