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@randrews
Created June 3, 2015 04:45
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Toy calculator in Lua, version 2
setmetatable(_ENV, { __index=lpeg })
VARS = {}
function eval(...)
local args = {...}
local accum = args[1]
for i = 2, #args, 2 do
local operator = args[i]
local num2 = args[i+1]
if operator == '+' then
accum = accum + num2
elseif operator == '-' then
accum = accum - num2
elseif operator == '*' then
accum = accum * num2
elseif operator == '/' then
accum = accum / num2
end
end
return accum
end
function assign(ref, value)
ref.scope[ref.index] = value
return value
end
function lookup(ref)
return ref.scope[ref.index]
end
function makeref(...)
local indices = {...}
local tbl = VARS
for i = 1, #indices-1 do
tbl = tbl[indices[i]]
end
return {scope=tbl, index=indices[#indices]}
end
spc = S(" \t\n")^0
digit = R('09')
number = C( (P("-") + digit) *
digit^0 *
( P('.') * digit^0 )^-1 ) / tonumber * spc
lparen = "(" * spc
rparen = ")" * spc
lbrack = "[" * spc
rbrack = "]" * spc
comma = "," * spc
expr_op = C( S('+-') ) * spc
term_op = C( S('*/') ) * spc
letter = R('AZ','az')
name = C( letter * (digit+letter+"_")^0 ) * spc
stmt = spc * P{
"STMT";
STMT =
V("REF") * "=" * spc * V("VAL") / assign +
V("EXPR"),
EXPR = V("TERM") * ( expr_op * V("TERM") )^0 / eval,
TERM = V("FACT") * ( term_op * V("FACT") )^0 / eval,
REF = name * (lbrack * V("EXPR") * rbrack)^0 / makeref,
FACT =
number / eval +
lparen * V("EXPR") * rparen / eval +
V("REF") / lookup,
ARRAY = lbrack * Ct( V("VAL_LIST")^-1 ) * rbrack,
VAL_LIST = V("VAL") * (comma * V("VAL"))^0,
VAL = V("EXPR") + V("ARRAY")
}
function test(expr)
assert(expr:match(" 1 + 2 ") == 3)
assert(expr:match("1+2+3+4+5") == 15)
assert(expr:match("2*3*4 + 5*6*7") == 234)
assert(expr:match(" 1 * 2 + 3") == 5)
assert(expr:match("( 2 +2) *6") == 24)
stmt:match("a=3"); assert(VARS.a == 3)
assert(stmt:match("a") == 3)
assert(stmt:match("a * 5") == 15); VARS.a=nil
stmt:match("a = [ 4, 5, 6 ]");
assert(VARS.a[1] == 4)
assert(VARS.a[2] == 5)
assert(VARS.a[3] == 6)
VARS.a=nil
stmt:match("b = [ ]");
assert(VARS.b[1] == nil)
VARS.b=nil
stmt:match("c = [[1,2], [3,4]]")
assert(VARS.c[1][1] == 1)
assert(VARS.c[1][2] == 2)
assert(VARS.c[2][1] == 3)
assert(VARS.c[2][2] == 4)
assert(stmt:match("c[4/2][1]") == 3)
stmt:match("c[3] = 5")
assert(VARS.c[3] == 5)
VARS.c=nil
end
function repl(file)
file = file or io.input()
parser = stmt
for line in file:lines() do
print(parser:match(line))
end
end
@achan001
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achan001 commented Aug 29, 2018

There is a reason why no programming language I know allows variable names to start with digits.

How about scheme ?

(define (2X value) (+ value value))

@eric-2
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eric-2 commented Aug 29, 2018

I know about scheme but I don`t know it, certainly not well enough to know that it allows identifiers to start with digits (or be completely made of digits?) but the syntax of scheme makes it easy enough to parse anyway...
S expressions were my first LPeg pattern to learn how to generate a nice ast.
But I was wrong: I know enough Forth, that I should have remembered that it allows this too.

@sourcevault
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Hmm...

There must be some pattern that accepts :

123a # variable 

123 # number

I am wondering why the compiler gods do not allow it though, JavaScript certainly throws an error.

I guess the benefit of allowing it does not outweigh the cost to the compiler writer.

@eric-2
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eric-2 commented Aug 30, 2018

I already gave the solution above but here an implementation of the two important patterns (number must be tried before variable):

number = l.P'-'^-1 * l.R'09'^1 * (l.P'.' * l.R'09'^1)^-1 * (l.S'eE' * l.S'+-'^-1 * l.R'09'^1)^-1 * -l.R('az','AZ', '__')
variable = `l.R('az','AZ','09','__')^1

The negative lookahead makes it possible.
The problem is, that it can get really confusing: '1e2' is a number but '1f2' and '1e' are names...
And it makes the parser a little more complicated for no real benefit.

@randrews
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Yeah, exactly, what eric-2 said. :)

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