Evaluating expression on tuple of variables doesn't work inside function

General Usage
2

Eval works in the global scope and can be tricky to use within functions. Changing your definition such that it creates a function with positional arguments gives a hint

julia> function eval_vars(expr::Expr, vars::NamedTuple)
           foo = eval(Expr(:->, Expr(:tuple, keys(vars)...), expr))
           @show methods(foo)
           return foo(values(vars)...)
       end
eval_vars (generic function with 1 method)

julia> eval_vars(ex, vs)
methods(foo) = # 1 method for anonymous function "#62":
[1] (::var"#62#63")(a, b) in Main
ERROR: MethodError: no method matching (::var"#62#63")(::Int64, ::Int64)
The applicable method may be too new: running in world age 32477, while current world is 32478.
Closest candidates are:
  (::var"#62#63")(::Any, ::Any) at none:0 (method too new to be called from this world context.)

I.e., the created function cannot be called immediately, but only after your function returns:

julia> function eval_vars(expr::Expr, vars::NamedTuple)
           return eval(Expr(:->, Expr(:parameters, keys(vars)...), expr))
       end
eval_vars (generic function with 1 method)

julia> eval_vars(ex, vs)(; vs...)
25

The simplest way to evaluate everything within a function, is to create an expression which directly computes the desired result when evaluated, e.g.,

julia> function eval_vars(expr::Expr, vars::NamedTuple)
           eval(:(let $(map((k, v) -> :($k = $v), keys(vars), values(vars))...); $expr end))
       end
eval_vars (generic function with 1 method)

julia> eval_vars(ex, vs)
25
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