Add a parser for BUGS

docs/src/parser.md

@@ -0,0 +1,13 @@
+Strictly speaking, the program is not "parsing", because the program doesn't output a syntax tree. 
+What is program does is take a token stream, with recursive descent structure, check the correctness of the program. 
+In the process of the recursive descent, BUGS syntax tokens will be translated into Julia syntax tokens. 
+The tokens that are already compatible with Julia will be remained, others will be either transformed or removed, also additional tokens may also be added.
+
+The parser will error given a program not in strict BUGS syntax.
+
+the general idea is:
+1. use `tokenize` to get the token vector
+2. inspect tokens and build the Julia version of the program in the form of a vector of tokens
+3. when it is appropriate to do so, just push the token to the Julia version of the program vector
+4. at the same time, some errors are detected and diagnostics are pushed to the diagnostics vector; also some tokens may be deleted, combined, or replaced 
+5. error recovery is very primitive: the heuristic is user forget something instead of put something wrong, a slightly more sophisticated approach is doing two versions: both "discard" and skip

src/graphs.jl

@@ -111,9 +111,14 @@ end
 # `_eval` mimic `eval` function, but use precompiled functions. This is possible because BUGS essentially only has
 # two kinds of expressions: function calls and indexing.
 # `env` is a dictionary mapping symbols in `expr` to values, values can be arrays or scalars
+function _eval(expr::Number, env)
+    return expr
+end
 function _eval(expr::Symbol, env)
     if expr == :nothing
         return nothing
+    elseif expr == :(:)
+        return Colon()
     else # intentional strict, all corner cases should be handled above
         return env[expr]
     end
@@ -137,7 +142,7 @@ function _eval(expr::Expr, env)
     end
 end
 function _eval(expr, env)
-    return expr
+    return error("Unknown expression type: $expr of type $(typeof(expr))")
 end
 
 """