Walk tree of x, making optimizations/fixes, and signaling SyntaxError.
(x, toplevel=False)
| 218 | ################ expand |
| 219 | |
| 220 | def expand(x, toplevel=False): |
| 221 | "Walk tree of x, making optimizations/fixes, and signaling SyntaxError." |
| 222 | require(x, x!=[]) # () => Error |
| 223 | if not isa(x, list): # constant => unchanged |
| 224 | return x |
| 225 | elif x[0] is _quote: # (quote exp) |
| 226 | require(x, len(x)==2) |
| 227 | return x |
| 228 | elif x[0] is _if: |
| 229 | if len(x)==3: x = x + [None] # (if t c) => (if t c None) |
| 230 | require(x, len(x)==4) |
| 231 | return list(map(expand, x)) |
| 232 | elif x[0] is _set: |
| 233 | require(x, len(x)==3); |
| 234 | var = x[1] # (set! non-var exp) => Error |
| 235 | require(x, isa(var, Symbol), "can set! only a symbol") |
| 236 | return [_set, var, expand(x[2])] |
| 237 | elif x[0] is _define or x[0] is _definemacro: |
| 238 | require(x, len(x)>=3) |
| 239 | _def, v, body = x[0], x[1], x[2:] |
| 240 | if isa(v, list) and v: # (define (f args) body) |
| 241 | f, args = v[0], v[1:] # => (define f (lambda (args) body)) |
| 242 | return expand([_def, f, [_lambda, args]+body]) |
| 243 | else: |
| 244 | require(x, len(x)==3) # (define non-var/list exp) => Error |
| 245 | require(x, isa(v, Symbol), "can define only a symbol") |
| 246 | exp = expand(x[2]) |
| 247 | if _def is _definemacro: |
| 248 | require(x, toplevel, "define-macro only allowed at top level") |
| 249 | proc = eval(exp) |
| 250 | require(x, callable(proc), "macro must be a procedure") |
| 251 | macro_table[v] = proc # (define-macro v proc) |
| 252 | return None # => None; add v:proc to macro_table |
| 253 | return [_define, v, exp] |
| 254 | elif x[0] is _begin: |
| 255 | if len(x)==1: return None # (begin) => None |
| 256 | else: return [expand(xi, toplevel) for xi in x] |
| 257 | elif x[0] is _lambda: # (lambda (x) e1 e2) |
| 258 | require(x, len(x)>=3) # => (lambda (x) (begin e1 e2)) |
| 259 | vars, body = x[1], x[2:] |
| 260 | require(x, (isa(vars, list) and all(isa(v, Symbol) for v in vars)) |
| 261 | or isa(vars, Symbol), "illegal lambda argument list") |
| 262 | exp = body[0] if len(body) == 1 else [_begin] + body |
| 263 | return [_lambda, vars, expand(exp)] |
| 264 | elif x[0] is _quasiquote: # `x => expand_quasiquote(x) |
| 265 | require(x, len(x)==2) |
| 266 | return expand_quasiquote(x[1]) |
| 267 | elif isa(x[0], Symbol) and x[0] in macro_table: |
| 268 | return expand(macro_table[x[0]](*x[1:]), toplevel) # (m arg...) |
| 269 | else: # => macroexpand if m isa macro |
| 270 | return list(map(expand, x)) # (f arg...) => expand each |
| 271 | |
| 272 | def require(x, predicate, msg="wrong length"): |
| 273 | "Signal a syntax error if predicate is false." |
no test coverage detected