In Julia v1.01 I would like to create a function from a string.
Background: In a numerical solver, a testcase is defined via a JSON file. It would be great if the user could specify the initial condition in string form.
This results in the following situation: Assume we have (from the JSON file)
fcn_as_string = "sin.(2*pi*x)"
Is there a way to convert this into a function fcn such that I can call
fcn(1.0) # = sin.(2*pi*1.0)
Performance is not really an issue, as the initial condition is evaluated once and then the actual computation consumes most of the time.
Can't get my code displayed correctly in a comment so here's a quick fix for crstnbr's solution
function fcnFromString(s)
f = eval(Meta.parse("x -> " * s))
return x -> Base.invokelatest(f, x)
end
function main()
s = "sin.(2*pi*x)"
f = fcnFromString(s)
f(1.)
end
julia> main()
-2.4492935982947064e-16
The functions Meta.parse and eval allow you to do this:
fcn_as_string = "sin.(2*pi*x)"
fcn = eval(Meta.parse("x -> " * fcn_as_string))
#show fcn(1.0)
This return -2.4492935982947064e-16 (due to rounding errors).
Related
Some of the parameters to a simulation I am writing are functions. When the output is generated, I want to put the definition of these functional parameters in the output. I have in mind a macro that somehow saves the definition as a string, and then defines it. For example, here's what I do now:
borda_score_fn(p) = exp(1/p)
global g_borda_score_fn_string = "exp(1/p)"
And then I write g_borda_score_fn_string to my output. But this is really ugly!
What I would like to do is something like this:
#paramfn borda_score_fn(p) = exp(1/p)
And later be able to both call borda_score_fn(p), and have the form (i.e., "exp(1/p)") available for writing to my output log. (The string form might get stashed in a global dict, actually, they both could.)
I have tried many version of this, but can't get the right set of parses and calls to get it to work. Any help would be appreciated.
This may be a bit different than what you have in mind, but one perhaps "Julian" approach might be to have the function itself return the form string via multiple dispatch, rather than defining a whole new global variable just for that. For example, say we have a type
struct Form end
that we can use for dispatch, then we can write
borda_score_fn(p) = exp(1/p)
borda_score_fn(::Form) = "exp(1/p)"
which can then be retrieved just by calling the function with our type
julia> borda_score_fn(2)
1.6487212707001282
julia> borda_score_fn(Form())
"exp(1/p)"
That might actually be not bad on its own. But, if you want a macro to do both parts at once, then something along the lines of
macro paramfn(e)
name = esc(e.args[1].args[1])
str = string(e.args[2].args[2])
f = esc(e)
quote
$name(::Form) = $str
$f
end
end
would let you write
julia> #paramfn borda_score_fn(p) = exp(1/p)
borda_score_fn (generic function with 2 methods)
julia> borda_score_fn(1)
2.718281828459045
julia> borda_score_fn(Form())
"exp(1 / p)"
For completeness, here's how you can do it in a way more similar to your original approach, but more idiomatically than with a global variable:
julia> module FormOf
export formof, #paramfn
function formof end
macro paramfn(expr)
name = esc(expr.args[1].args[1])
form_str = string(expr.args[2].args[2])
quote
$(esc(expr))
$FormOf.formof(::typeof($name)) = $form_str
$name
end
end
end
Main.FormOf
julia> FormOf.#paramfn borda_score_fn(p) = exp(1/p)
borda_score_fn (generic function with 1 method)
julia> FormOf.formof(borda_score_fn)
"exp(1 / p)"
However, since it defines a new method of FormOf.formof, this only works in global scope:
julia> function bla()
FormOf.#paramfn fn(p) = exp(1/p)
fn(10) + 1
end
ERROR: syntax: Global method definition around REPL[45]:10 needs to be placed at the top level, or use "eval".
Stacktrace:
[1] top-level scope
# REPL[50]:1
#cbk's solution does not have this limitation.
I have a script written in Lua 5.1 that imports third-party module and calls some functions from it. I would like to get a list of function calls from a module with their arguments (when they are known before execution).
So, I need to write another script which takes the source code of my first script, parses it, and extracts information from its code.
Consider the minimal example.
I have the following module:
local mod = {}
function mod.foo(a, ...)
print(a, ...)
end
return mod
And the following driver code:
local M = require "mod"
M.foo('a', 1)
M.foo('b')
What is the better way to retrieve the data with the "use" occurrences of the M.foo function?
Ideally, I would like to get the information with the name of the function being called and the values of its arguments. From the example code above, it would be enough to get the mapping like this: {'foo': [('a', 1), ('b')]}.
I'm not sure if Lua has functions for reflection to retrieve this information. So probably I'll need to use one of the existing parsers for Lua to get the complete AST and find the function calls I'm interested in.
Any other suggestions?
If you can not modify the files, you can read the files into a strings then parse mod file and find all functions in it, then use that information to parse the target file for all uses of the mod library
functions = {}
for func in modFile:gmatch("function mod%.(%w+)") do
functions[func] = {}
end
for func, call in targetFile:gmatch("M%.(%w+)%(([^%)]+)%)") do
args = {}
for arg in string.gmatch(call, "([^,]+)") do
table.insert(args, arg)
end
table.insert(functions[func], args)
end
Resulting table can then be serialized
['foo'] = {{"'a'", " 1"}, {"'b'"}}
3 possible gotchas:
M is not a very unique name and could vary possibly match unintended function calls to another library.
This example does not handle if there is a function call made inside the arg list. e.g. myfunc(getStuff(), true)
The resulting table does not know the typing of the args so they are all save as strings representations.
If modifying the target file is an option you can create a wrapper around your required module
function log(mod)
local calls = {}
local wrapper = {
__index = function(_, k)
if mod[k] then
return function(...)
calls[k] = calls[k] or {}
table.insert(calls[k], {...})
return mod[k](...)
end
end
end,
}
return setmetatable({},wrapper), calls
end
then you use this function like so.
local M, calls = log(require("mod"))
M.foo('a', 1)
M.foo('b')
If your module is not just functions you would need to handle that in the wrapper, this wrapper assumes all indexes are a function.
after all your calls you can serialize the calls table to get the history of all the calls made. For the example code the table looks like
{
['foo'] = {{'a', 1}, {'b'}}
}
I have the following struct (simplified), and some calculations done with this struct:
mutable struct XX{VecType}
v::VecType
end
long_calculation(x::XX) = sum(x.v)
as a part of the program i need to update the v value. the struct is callable and mainly used as a cache. here, the use of static arrays helps a lot in speeding up calculations, but the type of v is ultimately defined by an user. my problem lies when assigning new values to XX.v:
function (f::XX)(w)
f.v .= w #here lies the problem
return long_calculation(f)
this works if v <: Array and w is of any value, but it doesn't work when v <: StaticArrays.StaticArray, as setindex! is not defined on that type.
How can i write f.v .= w in a way that, when v allows it, performs an inplace modification, but when not, just creates a new value, and stores it in the XX struct?
There's a package for exactly this use case: BangBang.jl. From there, you can use setindex!!:
f.v = setindex!!(f.v, w)
Here I propose a simple solution that should be enough in most cases. Use multiple dispatch and define the following function:
my_assign!(f::XX, w) = (f.v .= w)
my_assign!(f::XX{<:StaticArray}, w) = (f.v = w)
and then simply call it in your code like this:
function (f::XX)(w)
my_assign!(f, w)
return long_calculation(f)
end
Then if you (or your users) get an error with a default implementation it is easy enough to add another method to my_assign! co cover other special cases when it throws an error.
Would such a solution be enough for you?
I'm trying to learn F# at the moment and have come up on a problem I can't solve and can't find any answers for on google.
Initially I wanted a log function that would work like the printf family of functions whereby I could provide a format string and a number of arguments (statically checked) but which would add a little metadata before printing it out. With googling, I found this was possible using a function like the following:
let LogToConsole level (format:Printf.TextWriterFormat<'T>) =
let extendedFormat = (Printf.TextWriterFormat<string->string->'T> ("%s %s: " + format.Value))
let date = DateTime.UtcNow.ToString "yyyy-MM-dd HH:mm:ss.fff"
let lvl = string level
printfn extendedFormat date lvl
having the printfn function as the last line of this function allows the varargs-like magic of the printf syntax whereby the partially-applied printfn method is returned to allow the caller to finish applying arguments.
However, if I have multiple such functions with the same signature, say LogToConsole, LogToFile and others, how could I write a function that would call them all keeping this partial-application magic?
Essential I'm looking for how I could implement a function MultiLog
that would allow me to call multiple printf-like functions from a single function call Such as in the ResultIWant function below:
type LogFunction<'T> = LogLevel -> Printf.TextWriterFormat<'T> -> 'T
let MultiLog<'T> (loggers:LogFunction<'T>[]) level (format:Printf.TextWriterFormat<'T>) :'T =
loggers
|> Seq.map (fun f -> f level format)
|> ?????????
let TheResultIWant =
let MyLog = MultiLog [LogToConsole; LogToFile]
MyLog INFO "Text written to %i outputs" 2
Perhaps the essence of this question can be caught more succintly: given a list of functions of the same signature how can I partially apply them all with the same arguments?
type ThreeArg = string -> int -> bool -> unit
let funcs: ThreeArg seq = [func1; func2; func3]
let MagicFunction = ?????
// I'd like this to be valid
let partiallyApplied = MagicFunction funcs "string"
// I'd also like this to be valid
let partiallyApplied = MagicFunction funcs "string" 255
// and this (fullyApplied will be `unit`)
let fullyApplied = MagicFunction funcs "string" 255 true
To answer the specific part of the question regarding string formatting, there is a useful function Printf.kprintf which lets you do what you need in a very simple way - the first parameter of the function is a continuation that gets called with the formatted string as an argument. In this continuation, you can just take the formatted string and write it to all the loggers you want. Here is a basic example:
let Loggers = [printfn "%s"]
let LogEverywhere level format =
Printf.kprintf (fun s ->
let date = DateTime.UtcNow.ToString "yyyy-MM-dd HH:mm:ss.fff"
let lvl = string level
for logger in Loggers do logger (sprintf "%s %s %s" date lvl s)) format
LogEverywhere "BAD" "hi %d" 42
I don't think there is a nice and simple way to do what you wanted to do in the more general case - I suspect you might be able to use some reflection or static member constraints magic, but fortunately, you don't need to in this case!
There is almost nothing to add to a perfect #TomasPetricek answer as he is basically a "semi-god" in F#. Another alternative, which comes to mind, is to use a computation expression (see, for example: https://fsharpforfunandprofit.com/series/computation-expressions.html). When used properly it does look like magic :) However, I have a feeling that it is a little bit too heavy for the problem, which you described.
Is there a way in Julia to specify that a function argument can take one of a set of values through type annotations? For example, let's say I have function foo which accepts a single argument
function foo(x::String)
print(x)
end
the argument x can only be a String. Is there a way to further constrain it in the function signature so that it can only be for example one of the strings "right", "left", or "center"?
In Julia, the motto should be "There's a type for that!".
One way of handling this would be to create a type with a constructor that only allows the values you want (and possibly stores them in a more efficient manner).
Here is one example:
const directions = ["left", "right", "center"]
immutable MyDirection
Direction::Int8
function MyDirection(str::AbstractString)
i = findnext(directions, str, 1)
i == 0 && throw(ArgumentError("Invalid direction string"))
return new(i)
end
end
Base.show(io::IO, x::MyDirection) = print(io, string("MyDirection(\"",directions[x.Direction],"\")"))
function foo(x::MyDirection)
println(x)
end
function foo(str::AbstractString)
x = MyDirection(str)
println(x)
end
test = MyDirection("left")
foo(test)
foo("right")
Note: my example is written with Julia 0.4!
Edit:
Another approach would be to use symbols, such as :left, :right, and :center,
instead of strings.
These have the advantage of being interned (so that they can be compared simply by comparing their address), and they can also be used directly for type parameters.
For example:
immutable MyDirection{Symbol} ; end
function MyDirection(dir::Symbol)
dir in (:left, :right, :center) || error("invalid direction")
MyDirection{dir}()
end
MyDirection(dir::AbstractString) = MyDirection(symbol(dir))
That will let you do things like:
x = MyDirection("left")
which will create an immutable object of type MyDirection{:left}.
No, it is not. That would be dispatching on values, which isn't possible in Julia.
I'm not sure what your actual application is, but there are some possibly-appropriate workarounds to this, e.g.
abstract Sam81Args
type ArgRight <:Sam81Args end
type ArgLeft <:Sam81Args end
type ArgCenter <:Sam81Args end
function foo{T<:Sam81Args}(x::Type{T})
println(T)
end
foo(ArgCenter)