let not implemented

[jyn514]

I found this because I ran into an infinite loop in bash_completion (the original, not the osh fork)!

The original code was

    local exclude flag i OPTIND=1
    # lots more code
    let "OPTIND += 1"

It looks like this should be the same as $(( )) but without then evaluating the result?

$ help let
let: let arg [arg ...]
    Evaluate arithmetic expressions.
    
    Evaluate each ARG as an arithmetic expression.  Evaluation is done in
    fixed-width integers with no check for overflow, though division by 0
    is trapped and flagged as an error.  The following list of operators is
    grouped into levels of equal-precedence operators.  The levels are listed
    in order of decreasing precedence.
    
    	id++, id--	variable post-increment, post-decrement
    	++id, --id	variable pre-increment, pre-decrement
    	-, +		unary minus, plus
    	!, ~		logical and bitwise negation
    	**		exponentiation
    	*, /, %		multiplication, division, remainder
    	+, -		addition, subtraction
    	<<, >>		left and right bitwise shifts
    	<=, >=, <, >	comparison
    	==, !=		equality, inequality
    	&		bitwise AND
    	^		bitwise XOR
    	|		bitwise OR
    	&&		logical AND
    	||		logical OR
    	expr ? expr : expr
    			conditional operator
    	=, *=, /=, %=,
    	+=, -=, <<=, >>=,
    	&=, ^=, |=	assignment
    
    Shell variables are allowed as operands.  The name of the variable
    is replaced by its value (coerced to a fixed-width integer) within
    an expression.  The variable need not have its integer attribute
    turned on to be used in an expression.
    
    Operators are evaluated in order of precedence.  Sub-expressions in
    parentheses are evaluated first and may override the precedence
    rules above.
    
    Exit Status:
    If the last ARG evaluates to 0, let returns 1; let returns 0 otherwise.

[andychu]

Yup let is just a synonym for (( LHS = expr )) ... It's low priority since the the rewrite is trivial, and so far I only ran into it in bash-completion. But it would be pretty easy to implement in any case.

over time we can adjust this: https://github.com/oilshell/oil/wiki/What-Is-Expected-to-Run-Under-OSH