[julia-users] How to define rounding macros depending on type

[David P. Sanders]

Hi,

For the ValidatedNumerics package, I am trying to define macros for 
rounding that change the corresponding rounding mode depending on the type 
of their argument, Float64 or BigFloat.
(Currently only BigFloats are used, but Float64s are much faster.)

The following seems to work, but it also seems to be tricky and fragile. Is 
there a better way?

```
function set_rounding_mode(expr, rounding_mode)
    value = @eval($expr)
    T = typeof(value)
    :(set_rounding($T, $rounding_mode))
end

macro round_down(expr)
    round_expr = set_rounding_mode(expr, RoundDown)
    expr = :($round_expr; $expr)
end

@round_down(0.1)
```

However, if I now want to reset the rounding mode after performing the 
operation, as follows

```
macro round_down(expr)
    round_expr1 = set_rounding_mode(expr, RoundDown)
    round_expr2 = set_rounding_mode(expr, RoundNearest)
    expr = :($round_expr1; $expr; $round_expr2)
end

@round_down(0.1)
```

it returns 0, since the second call to set_rounding_mode returns the 
rounding mode, which is 0.

Surely I am missing some subtle issues about macro hygiene, escaping, and 
such like. 
Please help!

Thanks,
David.