Making a conditional inner function assignment - julia

I want to make a conditional funtion assignment inside a function (by calling function bar1 or bar2) based on its arguments. The thing is, it must assign bar1 or bar2 to a one argument function.
This is an example of what I am doing:
function foo(a, b=false)
if b
f(t) = t*a; #bar1(t,a)
else
f(t) = t/a; #bar2(t,a)
end
return f(2.0);
end
Calling
foo(1.1, true)
Outputs
1.8181818181818181
Calling
foo(1.1, false)
Outputs
ERROR: UndefVarError: f not defined
Stacktrace:
[1] foo(::Float64, ::Bool) at C:\Users\pedro\Documents\codigos\Julia_HEM\compara_hem_neumann.jl:89
[2] top-level scope at none:0
Why is f not defined when b=false?
Edit
A workaround is to define, for each condition, a different function. For example
function foo(a, b=false)
if b
f(t) = t*a; #bar1(t,a)
return f(2.0);
else
g(t) = t/a; #bar2(t,a)
return g(2.0);
end
end
The question remains, though...

This is an open issue in Julia language (see https://github.com/JuliaLang/julia/issues/15602). It is recommended to use an anonymous function in such cases.
function foo(a, b=false)
if b
f = t -> t*a
else
f = t -> t/a
end
return f(2.0)
end
Note though that this will not be very efficient as the compiler is not able to infer the return type of foo (so probably you would have to add a declaration of function return type). Your answer in EDIT is type stable.

Related

Changing the input (i.e. x+2y) of a macro to an expression ( :(x+2y)), How to produce the same output?

The code at the end of this post constructs a function which is bound to the variables of a given dictionary. Furthermore, the function is not bound to the actual name of the dictionary (as I use the Ref() statement).
An example:
julia> D = Dict(:x => 4, :y => 5)
julia> f= #mymacro4(x+2y, D)
julia> f()
14
julia> DD = D
julia> D = nothing
julia> f()
14
julia> DD[:x] = 12
julia> f()
22
Now I want to be able to construct exactly the same function when I only have access to the expression expr = :(x+2y).
How do I do this? I tried several things, but was not able to find a solution.
julia> f = #mymacro4(:(x+2y), D)
julia> f() ### the function evaluation should also yield 14. But it yields:
:(DR.x[:x] + 2 * DR.x[:y])
(I actually want to use it within another macro in which the dictionary is automatically created. I want to store this dictionary and the function within a struct, such that I'm able to call this function at a later point in time and manipulate the objects in the dictionary. If necessary, I may post the complete example and explain the complete problem.)
_freevars2(literal) = literal
function _freevars2(s::Symbol)
try
if typeof(eval(s)) <: Function
return s
else
return Meta.parse("DR.x[:$s]")
end
catch
return Meta.parse("DR.x[:$s]")
end
end
function _freevars2(expr::Expr)
for (it, s) in enumerate(expr.args)
expr.args[it] = _freevars2(s)
end
return expr
end
macro mymacro4(expr, D)
expr2 = _freevars2(expr)
quote
let DR = Ref($(esc(D)))
function mysym()
$expr2
end
end
end
end

Keyword Arguments - Functions

I am a beginner in Julia,
how can I create functions with keywords for arguments without having to initialize these arguments in function?
A very simple example:
function f(;a = 1, b = 2)
a+b
end
I would like to do:
function f(;a, b)
a+b
end
Best regards.
This is a new feature in version 0.7 — you can actually write it just as you'd like.
Julia's syntax on versions 0.6 and prior require you to give them a default value, but since that default value is evaluated at call time, you can actually use an error function to require them:
julia> function f(;a=error("a not provided"), b=error("b not provided"))
a+b
end
f (generic function with 1 method)
julia> f()
ERROR: a not provided
Stacktrace:
[1] f() at ./REPL[1]:2
julia> f(a=2)
ERROR: b not provided
Stacktrace:
[1] (::#kw##f)(::Array{Any,1}, ::#f) at ./<missing>:0
julia> f(a=2, b=3)
5
This is comming in Julia 0.7 line:
Keyword arguments can be required: if a default value is omitted, then an exception is thrown if the caller does not assign the keyword a value (#25830).
So:
function f(;a, b)
a+b
end
Will become syntax sugar for:
function f(;a = throw(UndefKeywordError(:a)), b = throw(UndefKeywordError(:b)))
a+b
end
Another workaround is to create a function with variadic keyword arguments and leave any requirements over the expected keyword inputs as assertions inside the code. E.g.
function f( ; kwargs... )
V = Dict( kwargs )
try; assert( haskey( V, :a ) ); assert( haskey( V, :b ) )
catch e; throw( AssertionError("KWargs need to be a and b") )
end
V[:a] + V[:b]
end
f(a=1, b=2) #> 3
f(a=1, c=2) #> ERROR: AssertionError: KWargs need to be a and b
Or even as simple as:
function f( ; kwargs... )
V = Dict( kwargs )
a = V[:a]
b = V[:b]
a + b
end
f(a=1, c=2) #> ERROR: KeyError: key :b not found
Disclaimer: I'm not recommending this, I'm just saying it's another workaround to consider depending on what functionality you have in mind.

Evaluate expression with local variables

I'm writing a genetic program in order to test the fitness of randomly generated expressions. Shown here is the function to generate the expression as well a the main function. DIV and GT are defined elsewhere in the code:
function create_single_full_tree(depth, fs, ts)
"""
Creates a single AST with full depth
Inputs
depth Current depth of tree. Initially called from main() with max depth
fs Function Set - Array of allowed functions
ts Terminal Set - Array of allowed terminal values
Output
Full AST of typeof()==Expr
"""
# If we are at the bottom
if depth == 1
# End of tree, return function with two terminal nodes
return Expr(:call, fs[rand(1:length(fs))], ts[rand(1:length(ts))], ts[rand(1:length(ts))])
else
# Not end of expression, recurively go back through and create functions for each new node
return Expr(:call, fs[rand(1:length(fs))], create_single_full_tree(depth-1, fs, ts), create_single_full_tree(depth-1, fs, ts))
end
end
function main()
"""
Main function
"""
# Define functional and terminal sets
fs = [:+, :-, :DIV, :GT]
ts = [:x, :v, -1]
# Create the tree
ast = create_single_full_tree(4, fs, ts)
#println(typeof(ast))
#println(ast)
#println(dump(ast))
x = 1
v = 1
eval(ast) # Error out unless x and v are globals
end
main()
I am generating a random expression based on certain allowed functions and variables. As seen in the code, the expression can only have symbols x and v, as well as the value -1. I will need to test the expression with a variety of x and v values; here I am just using x=1 and v=1 to test the code.
The expression is being returned correctly, however, eval() can only be used with global variables, so it will error out when run unless I declare x and v to be global (ERROR: LoadError: UndefVarError: x not defined). I would like to avoid globals if possible. Is there a better way to generate and evaluate these generated expressions with locally defined variables?
Here is an example for generating an (anonymous) function. The result of eval can be called as a function and your variable can be passed as parameters:
myfun = eval(Expr(:->,:x, Expr(:block, Expr(:call,:*,3,:x) )))
myfun(14)
# returns 42
The dump function is very useful to inspect the expression that the parsers has created. For two input arguments you would use a tuple for example as args[1]:
julia> dump(parse("(x,y) -> 3x + y"))
Expr
head: Symbol ->
args: Array{Any}((2,))
1: Expr
head: Symbol tuple
args: Array{Any}((2,))
1: Symbol x
2: Symbol y
typ: Any
2: Expr
[...]
Does this help?
In the Metaprogramming part of the Julia documentation, there is a sentence under the eval() and effects section which says
Every module has its own eval() function that evaluates expressions in its global scope.
Similarly, the REPL help ?eval will give you, on Julia 0.6.2, the following help:
Evaluate an expression in the given module and return the result. Every Module (except those defined with baremodule) has its own 1-argument definition of eval, which evaluates expressions in that module.
I assume, you are working in the Main module in your example. That's why you need to have the globals defined there. For your problem, you can use macros and interpolate the values of x and y directly inside the macro.
A minimal working example would be:
macro eval_line(a, b, x)
isa(a, Real) || (warn("$a is not a real number."); return :(throw(DomainError())))
isa(b, Real) || (warn("$b is not a real number."); return :(throw(DomainError())))
return :($a * $x + $b) # interpolate the variables
end
Here, #eval_line macro does the following:
Main> #macroexpand #eval_line(5, 6, 2)
:(5 * 2 + 6)
As you can see, the values of macro's arguments are interpolated inside the macro and the expression is given to the user accordingly. When the user does not behave,
Main> #macroexpand #eval_line([1,2,3], 7, 8)
WARNING: [1, 2, 3] is not a real number.
:((Main.throw)((Main.DomainError)()))
a user-friendly warning message is provided to the user at parse-time, and a DomainError is thrown at run-time.
Of course, you can do these things within your functions, again by interpolating the variables --- you do not need to use macros. However, what you would like to achieve in the end is to combine eval with the output of a function that returns Expr. This is what the macro functionality is for. Finally, you would simply call your macros with an # sign preceding the macro name:
Main> #eval_line(5, 6, 2)
16
Main> #eval_line([1,2,3], 7, 8)
WARNING: [1, 2, 3] is not a real number.
ERROR: DomainError:
Stacktrace:
[1] eval(::Module, ::Any) at ./boot.jl:235
EDIT 1. You can take this one step further, and create functions accordingly:
macro define_lines(linedefs)
for (name, a, b) in eval(linedefs)
ex = quote
function $(Symbol(name))(x) # interpolate name
return $a * x + $b # interpolate a and b here
end
end
eval(ex) # evaluate the function definition expression in the module
end
end
Then, you can call this macro to create different line definitions in the form of functions to be called later on:
#define_lines([
("identity_line", 1, 0);
("null_line", 0, 0);
("unit_shift", 0, 1)
])
identity_line(5) # returns 5
null_line(5) # returns 0
unit_shift(5) # returns 1
EDIT 2. You can, I guess, achieve what you would like to achieve by using a macro similar to that below:
macro random_oper(depth, fs, ts)
operations = eval(fs)
oper = operations[rand(1:length(operations))]
terminals = eval(ts)
ts = terminals[rand(1:length(terminals), 2)]
ex = :($oper($ts...))
for d in 2:depth
oper = operations[rand(1:length(operations))]
t = terminals[rand(1:length(terminals))]
ex = :($oper($ex, $t))
end
return ex
end
which will give the following, for instance:
Main> #macroexpand #random_oper(1, [+, -, /], [1,2,3])
:((-)([3, 3]...))
Main> #macroexpand #random_oper(2, [+, -, /], [1,2,3])
:((+)((-)([2, 3]...), 3))
Thanks Arda for the thorough response! This helped, but part of me thinks there may be a better way to do this as it seems too roundabout. Since I am writing a genetic program, I will need to create 500 of these ASTs, all with random functions and terminals from a set of allowed functions and terminals (fs and ts in the code). I will also need to test each function with 20 different values of x and v.
In order to accomplish this with the information you have given, I have come up with the following macro:
macro create_function(defs)
for name in eval(defs)
ex = quote
function $(Symbol(name))(x,v)
fs = [:+, :-, :DIV, :GT]
ts = [x,v,-1]
return create_single_full_tree(4, fs, ts)
end
end
eval(ex)
end
end
I can then supply a list of 500 random function names in my main() function, such as ["func1, func2, func3,.....". Which I can eval with any x and v values in my main function. This has solved my issue, however, this seems to be a very roundabout way of doing this, and may make it difficult to evolve each AST with each iteration.

Check if a function has keywords arguments in Julia

Is there a way to check if a function has keywords arguments in Julia? I am looking for something like has_kwargs(fun::Function) that would return true if fun has a method with keyword arguments.
The high level idea is to build a function:
function master_fun(foo::Any, fun::Function, ar::Tuple, kw::Tuple)
if has_kwargs(fun)
fun(ar... ; kw...)
else
fun(ar...)
end
end
Basically, #Michael K. Borregaard's suggestion to use try-catch is correct and officially works.
Looking into the unofficial implementation details, I came up with the followng:
haskw(f,tup) = isdefined(typeof(f).name.mt,:kwsorter) &&
length(methods(typeof(f).name.mt.kwsorter,(Vector{Any},typeof(f),tup...)))>0
This function first looks if there is any keyword processing on any method of the generic function, and if so, looks at the specific tuple of types.
For example:
julia> f(x::Int) = 1
f (generic function with 1 method)
julia> f(x::String ; y="value") = 2
f (generic function with 2 methods)
julia> haskw(f,(Int,))
false
julia> haskw(f,(String,))
true
This should be tested for the specific application, as it probably doesn't work when non-leaf types are involved. As Michael commented, in the question's context the statement would be:
if haskw(fun, typeof.(ar))
...
I don't think you can guarantee that a given function has keyword arguments. Check
f(;x = 3) = println(x)
f(x) = println(2x)
f(3)
#6
f(x = 3)
#3
f(3, x = 3)
#ERROR: MethodError: no method matching f(::Int64; x=3)
#Closest candidates are:
# f(::Any) at REPL[2]:1 got unsupported keyword argument "x"
# f(; x) at REPL[1]:1
So, does the f function have keywords? You can only check for a given method. Note that, in your example above, you'd normally just do
function master_fun(foo, fun::Function, ar::Tuple, kw....)
fun(ar... ; kw...)
end
which should work, and if keywords are passed to a function that does not take them you'd just leave the error reporting to fun. If that is not acceptable you could try to wrap the fun(ar...; kw...) in a try-catch block.

How to make type attribute be a function of other type attributes?

To declare a new composite type, we use the following syntax
type foo
a::Int64
b::Int64
end
and instantiate like such
x = foo(1,3)
Is there some way to have type attributes that always just a function of other attributes? For example, is there some way to do the following (which is invalid syntax)...
type foo
a::Int64
b::Int64
c = a + b
end
My current workaround is just to define a function which calculates c and returns an instance of the type, like so...
type foo
a::Int64
b::Int64
c::Int64
end
function foo_maker(a, b)
return foo(a, b, a+b)
end
Is there a more elegant solution? Possibly one that can be contained within the type definition?
EDIT - 3/7/14
With Cristóvão's suggestion in mind, I've ended up declaring constructors like the following to allow for keyword args and attributes calculated upon instantiation
# Type with optional keyword argument structure
type LargeType
# Declare all the attributes in order up top
q::Int64
w::Int64
e::Int64
r::Int64
t::Int64
y::Int64
a::Number
b::Number
c::Number
# Declare Longer constructor with stuff going on in the body
LargeType(;q=1,w=1,e=1,r=1,t=1,y=1) = begin
# Large Constructor Example
a = round(r^t - log(pi))
b = a % t
c = a*b
# Return new instance with correctly ordered arguments
return new(q,w,e,r,t,y,a,b,c)
end
end
println(LargeType(r=2,t=5))
Try this:
julia> type foo
a::Int64
b::Int64
c::Int64
foo(a::Int64, b::Int64) = new(a, b, a+b)
end
julia> foo(1,2)
foo(1,2,3)
julia> foo(4,5,6)
no method foo(Int64, Int64, Int64)
However, that won't prevent one from manually changing a, b or c and rendering c inconsistent. To prevent that, if it presents no other problems, you can make foo immutable:
julia> immutable foo
...
There isn't any way to do this currently, but there might be in the future:
https://github.com/JuliaLang/julia/issues/1974

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