I have some code here:
require "math"
local dozenDonuts
local oneDonut
local function roundToFirstDecimal(t)
return math.round(t*10)*0.1
end
When I run the code above, I get the following error:
lua: f:\my codes\donut.lua:6: attempt to call field 'round' (a nil value)
stack traceback:
f:\my codes\donut.lua:6: in function 'roundToFirstDecimal'
f:\my codes\donut.lua:17: in main chunk
[C]: ?
Is round not an attribute of math? How can I round to the first decimal?
math.round is not part of the standard Lua math library. But it is simple to write:
function math.round(x)
return math.floor(x+0.5)
end
Related
If I have a method
macro doarray(arr)
if in(:head, fieldnames(typeof(arr))) && arr.head == :vect
println("A Vector")
else
throw(ArgumentError("$(arr) should be a vector"))
end
end
it works if I write this
#doarray([x])
or
#doarray([:x])
but the following code rightly does not work, raising the ArgumentError(i.e. ArgumentError: alist should be a vector).
alist = [:x]
#doarray(alist)
How can I make the above to act similarly as #doarray([x])
Motivation:
I have a recursive macro(say mymacro) which takes a vector, operates on the first value and then calls recursively mymacro with the rest of the vector(say rest_vector). I can create rest_vector, print the value correctly(for debugging) but I don't know how to evaluate rest_vector when I feed it to the mymacro again.
EDIT 1:
I'm trying to implement logic programming in Julia, namely MiniKanren. In the Clojure implementation that I am basing this off, the code is such.
(defmacro fresh
[var-vec & clauses]
(if (empty? var-vec)
`(lconj+ ~#clauses)
`(call-fresh (fn [~(first var-vec)]
(fresh [~#(rest var-vec)]
~#clauses)))))
My failing Julia code based on that is below. I apologize if it does not make sense as I am trying to understand macros by implementing it.
macro fresh(varvec, clauses...)
if isempty(varvec.args)
:(lconjplus($(esc(clauses))))
else
varvecrest = varvec.args[2:end]
return quote
fn = $(esc(varvec.args[1])) -> #fresh($(varvecvest), $(esc(clauses)))
callfresh(fn)
end
end
end
The error I get when I run the code #fresh([x, y], ===(x, 42))(you can disregard ===(x, 42) for this discussion)
ERROR: LoadError: LoadError: UndefVarError: varvecvest not defined
The problem line is fn = $(esc(varvec.args[1])) -> #fresh($(varvecvest), $(esc(clauses)))
If I understand your problem correctly it is better to call a function (not a macro) inside a macro that will operate on AST passed to the macro. Here is a simple example how you could do it:
function recarray(arr)
println("head: ", popfirst!(arr.args))
isempty(arr.args) || recarray(arr)
end
macro doarray(arr)
if in(:head, fieldnames(typeof(arr))) && arr.head == :vect
println("A Vector")
recarray(arr)
else
throw(ArgumentError("$(arr) should be a vector"))
end
end
Of course in this example we do not do anything useful. If you specified what exactly you want to achieve then I might suggest something more specific.
Just messing with elixir result got an error that I cannot figure out.Here is my snippet,I implemented a simple Parallel map function for some api calls. func is the function where the actual calls are made and it returns {:ok,result} or {:error,reason} which I handle after the mapping in a different function
Originally
def pmap(collection,func,limit \\ 5000) do
collection
|> Enum.map(&Task.async(func.(&1)))
|> Enum.map(&Task.await(&1,limit))
Got the error so changed it to this for readability
def pmap(collection,func,limit) do
collection
|>Enum.map(fn(x) -> Task.async(func.(x)) end)
|>Enum.map(fn(task) -> Task.await(task,limit) end)
The error I am getting states
[error] Task #PID<0.197.0> started from #PID<0.187.0> terminating
** (BadFunctionError) expected a function, got: {:ok,result}
erlang.apply/2
(elixir) lib/task/supervised.ex:85: Task.Supervised.do_apply/2
(elixir) lib/task/supervised.ex:36: Task.Supervised.reply/5
(stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Function: &:erlang.apply/2
From what I gather it is assuming that the actual task collection element is being passed as the function
So I modified the function to
IO.puts "PMAP BEGUN"
tasks = collection
|> Enum.map(fn(x) -> Task.async(func.(x)) end)
answer = Enum.map(tasks,fn(task) -> Task.await(task,limit) end)
IO.puts "PMAP DONE"
answer
The IO.puts were for debugging.So I guess the error happens on the second map as PMAP DONE is never displayed. I still have the same error.
What exactly is wrong here? I have written the same function before almost verbatim and it worked.
The problem you're experiencing happens on line:
|> Enum.map(fn(x) -> Task.async(func.(x)) end)
to make it work you need to wrap function execution with anonymous function, like:
|> Enum.map(fn(x) -> Task.async(fn -> func.(x) end) end)
This due to fact if you run func.(x) this will evaluate, and the result of evaluation will be passed to Task.async/1, while when you wrap it with function, it will be up to Task to execute it.
The hint is indicated in error message:
** (BadFunctionError) expected a function, got: {:ok,result}
Hope this helps!
What is the best method for handling a Void type when it is returned by a function? The suggestions in http://docs.julialang.org/en/release-0.5/manual/faq/#how-does-null-or-nothingness-work-in-julia don't work.
A MWE (must be run from the REPL so Base.source_dir() returns Void):
julia> isempty(Base.source_dir())
ERROR: MethodError: no method matching start(::Void)
Closest candidates are:
start(::SimpleVector) at essentials.jl:170
start(::Base.MethodList) at reflection.jl:258
start(::IntSet) at intset.jl:184
...
in isempty(::Void) at ./iterator.jl:3
in isempty(::Void) at /Applications/Julia-0.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?
julia> isdefined(Base.source_dir())
ERROR: TypeError: isdefined: expected Symbol, got Void
julia> typeof(Base.source_dir()) == Void
true
This is on Julia 0.5. The latter option works, but it's a bit ugly.
Void is a singleton -- a type with exactly one instance.
That one instance is Void() also called nothing.
Be aware that nothing === Void()
You can treat it just like any other value.
It is returned by a bunch of functions, like println.
You can check if something has returned nothing -- ie and instance of type Void.
By
julia> println()===nothing
true
For the sake of type-stability,
a method should not return nothing some of the time, and something some of the time.
in those case it should instead return a Nullable,
generally.
I am currently learning sml but I have one question that I can not find an answer for. I have googled but still have not found anything.
This is my code:
fun diamond(n) =
if(n=1) then (
print("*")
) else (
print("*")
diamond(n-1)
)
diamond(5);
That does not work. I want the code to show as many * as number n is and I want to do that with recursion, but I don't understand how to do that.
I get an error when I try to run that code. This is the error:
Standard ML of New Jersey v110.78 [built: Thu Aug 20 19:23:18 2015]
[opening a4_p2.sml] a4_p2.sml:8.5-9.17 Error: operator is not a
function [tycon mismatch] operator: unit in expression:
(print "*") diamond /usr/local/bin/sml: Fatal error -- Uncaught exception Error with 0 raised at
../compiler/TopLevel/interact/evalloop.sml:66.19-66.27
Thank you
You can do side effects in ML by using ';'
It will evaluate whatever is before the ';' and discard its result.
fun diamond(n) =
if(n=1)
then (print "*"; 1)
else (print "*"; diamond(n-1));
diamond(5);
The reason for the error is because ML is a strongly typed language that although you don't need to specify types explicitly, it will infer them based on environmental factors at compile time. For this reason, every evaluation of functions, statements like if else need to evaluate to an unambiguous singular type.
If you were allowed to do the following:
if(n=1)
then 1
else print "*";
then the compiler will get a different typing for the then and else branch respectively.
For the then branch the type would be int -> int whereas the type for the else branch would be int -> unit
Such a dichotomy is not allowed under a strongly typed language.
As you need to evaluate to a singular type, you will understand that ML does not support the execution of a block of instructions as we commonly see in other paradigms which transposed to ML naively would render something like this:
....
if(n=1)
then (print "1"
print "2"
)
else (print "3"
diamond(n-1)
)
...
because what type would the then branch evaluate to? int -> unit? Then what about the other print statement? A statement has to return a singular result(even it be a compound) so that would not make sense. What about int -> unit * unit? No problem with that except that syntactically speaking, you failed to communicate a tuple to the compiler.
For this reason, the following WOULD work:
fun diamond(n) =
if(n=1)
then (print "a", 1) /* A tuple of the type unit * int */
else diamond(n-1);
diamond(5);
As in this case you have a function of type int -> unit * int.
So in order to satisfy the requirement of the paradigm of strongly typed functional programming where we strive for building mechanisms that evaluate to one result-type, we thus need to communicate to the compiler that certain statements are to be executed as instructions and are not to be incorporated under the typing of the function under consideration.
For this reason, you use ';' to communicate to the compiler to simply evaluate that statement and discard its result from being incorporated under the type evaluation of the function.
As far as your actual objective is concerned, following is a better way of writing the function, diamond as type int -> string:
fun diamond(n) =
if(n=1)
then "*"
else "*" ^ diamond(n-1);
print( diamond(5) );
The above way is more for debugging purposes.
Im new on this project and am going to write, what i thought was a simple thing. A recursive function that writes nested xml elements in x levels (denoted by a variable). So far I have come up with this, but keeps getting a compile error. Please note that i have to generate new xml , not query existing xml:
xquery version "1.0";
declare function local:PrintTest($amount)
{
<test>
{
let $counter := 0
if ($counter <= $amount )
then local:PrintTest($counter)
else return
$counter := $counter +1
}
</test>
};
local:PrintPerson(3)
My error is:
File Untitled1.xquery: XQuery transformation failed
XQuery Execution Error!
Unexpected token - " ($counter <= $amount ) t"
I never understood xquery, and cant quite see why this is not working (is it just me or are there amazingly few resources on the Internet concerning XQuery?)
You have written this function in a procedural manner, XQuery is a functional language.
Each function body can only be a single expression; it looks like you are trying to write statements (which do not exist in XQuery).
Firstly, your let expression must be followed by a return keyword.
return is only used as part of a FLWOR expression, a function always evaluates to a value. As you have written it return is equivalent to /return and so will return a node called return.
The line $counter := $counter + 1 is not valid XQuery at all. You can only set a variable like this with a let expression, and in this case it would create a new variable called counter which replaced the old one, that would be in scope only in the return expression of the variable.
The correct way to do what you are trying to do is to reduce the value of $argument each time the function recurses, and stop when you hit 0.
declare function local:Test($amount)
{
if ($amount == 0)
then ()
else
<test>
{
local:Test($amount - 1)
}
</test>
};
local:Test(3)
Note that I have changed the name of the function to Test. The name "PrintTest" was misleading, as this implies that the function does something (namely, printing). The function in fact just returns a node, it does not do any printing. In a purely functional langauge (which XQuery is quite close to) a function never has any side effects, it merely returns a value (or in this case a node).
The line $counter := $counter + 1 is valid XQuery Scripting.