I've read that every form in Common Lisp returns something when evaluated. However, recently I've been playing with ASDF API and found a function that returns nothing:
CL-USER> (asdf:clear-output-translations)
; No value
How is this possible and why doesn't it return something like NIL?
Common Lisp allows functions to return from 0 upto MULTIPLE-VALUES-LIMIT values. The constant MULTIPLE-VALUES-LIMIT is 20 or larger.
The function VALUES allows one to return multiple values, including zero values.
Thus a common idiom is to use the form (values) when a function has no useful return value and is just called for side effects. Also this usually causes the Lisp listener (aka REPL) to not print anything as return value, which can be useful for aesthetic reasons.
Note that variables only have a single value and that one can bind only exactly one value to a variable.
Function can return no values by using (values) form.
For example:
(defun foo ()) ;; returns nil
(defun bar () (values)) ;; returns nothing
Related
I want to check a function definition exists in a lisp program or not to decide which program block to run.
The function definition is written on another file with.Net & I am working for AutoCAD.
Please help.
There are many ways to do this, but ultimately you need to check whether the symbol corresponding to the function name holds a value (for example using the boundp function), and perhaps additionally whether such value is of SUBR, USUBR, or EXRXSUBR data type (using the type function).
For example:
(member (type YourFunctionName) '(subr usubr exrxsubr))
In this case, if the symbol YourFunctionName is null, (type YourFunctionName) will return nil which will cause the member expression to return nil. Similarly, if the value held by the YourFunctionName symbol is anything other than a function, the member function will return nil.
Since any non-nil value in AutoLISP is interpreted as True, the use of member will validate an if test expression, even though member does not explicitly return a boolean value.
Lee's Answer is great, many time to check function is loaded or not I am use (and functionName) it return T if exist or if not returns Nil.
Below is a sample code that addresses the problem I am having. The error message I am getting is
Function result 'sample' at (1) has no IMPLICIT type.
I label where line (1) is below.
I tried to follow this other question, however I wasn't able to figure it out. This function is within a module in my program and I made sure that the module has contains and I end the module after this function.
I also use implicit none in this function so I'm not sure why I get this message. How can I fix this error message?
Adding Real or Complex in front of function works, but I don't really get why. Shouldn't I only be able to use complex since the arrays are complex inside the function? Which is more suitable for my actual function? Both yield no compilation errors.
real function Sample(func) !this is line (1)
!complex function Sample(func)
implicit none
integer :: n,m
real :: x,y
complex, dimension(-9:9,-9:9), intent(in) :: func
complex, dimension(-9:9,-9:9) :: LocalF
LocalF = func
do n=-9,9
do m=-9,9
x = real(n)*0.2
y = real(m)*0.2
LocalF(n,m)= cmplx(z1(x,y),z2(x,y)) !assume z1,z2 are well defined
end do
end do
end function Sample
In Fortran every function has a result. If you like you can think of the result as a value returned by the function. Like every other value in a Fortran program a function result has a type, and a kind and a rank too.
By default the function result has the same name as the function itself, and its declaration is prepended to the function declaration. For example, here
integer function add(m,n)
integer, intent(in) :: a,b
add = a+b
end function
the function is called add and you can see (a) that the result is of type integer (and of default kind and scalar) and (b) that the result is formed by adding the two arguments together.
For functions returning arrays this syntax is not available, so you couldn't write something like
integer(1:4) add_vec(m1,m2)
In such cases you have to explicitly define the name (and later type and kind) of the result variable. Sticking with the simple example, something like
function add(m,n) result(addvec)
integer, intent(in) :: a(4),b(4)
integer, dimension(4) :: addvec
....
end function
Notice that you don't define the intent of the result.
In OP's case sample is, I think, intended to return a rank-2 array of complex values. I think OP needs to replace
function Sample(func) !this is line (1)
with
function Sample(func) result(LocalF)
and see how that goes. Here, if it is not evident already, you learn that the result name doesn't have to be the same as the name of the function.
Furthermore ... Adding Real or Complex in front of function works, but I don't really get why.
It might work in the sense of compiling, but executing it will lead to tears. By telling the compiler that the function result is either a real or complex value you satisfy the syntactical requirements for a function definition. But without assigning a (real or complex as declared) value to the result variable (called Sample in OP's code) the function will, at best, return junk.
To be as clear as I can ... in OP's original code there were two serious mistakes:
The function (result) was not given an explicit type, which lead to the compiler message shown.
The function did not include setting the value of the result variable, i.e. the variable with the same name as the function (in the absence of the result clause).
Procedures in Fortran come in two types: functions and subroutines. This question is about functions, so I'll consider just those.
What was missing in the first revision, giving the error about the implicit type of the function result1, was the result type.
Adding real function ... or complex function ..., etc., resolves that problem by explicitly giving the type of the function result. The linked documentation gives other ways of doing that.
The function's result is used when the function is referenced. When we have a reference like
func0 = Sample(func)
in the main program, the function Sample is invoked and the function result is defined in its execution. At the end of the function's execution its result is placed in the expression of the reference.
So, if you declare
real function Sample(func)
or
complex function Sample(func)
what you are saying is that the function result is either a real or complex entity. And when the function is evaluated, whatever value Sample had at the end is used in the expression (here assignment).
As a consequence of the function result being returned through Sample (in this case) we need to define its value. The important thing to note for the question, then, is that LocalF is a variable local to the function. If you mean it to be the result of the function you need to use the function result.
You have a number of options:
function Sample(func)
<type>, <attributes> :: sample ! Instead of LocalF
... :: func
end function
or
function Sample(func) result(LocalF)
<type>, <attributes> :: LocalF
... :: func
end function
You can even have
<type> function Sample(func)
<attribute statements for Sample>
... func
end function
but I really suggest you avoid that last one.
1 Note the error here is about type for the function result; in the linked question simply about the function when referenced.
Below is a sample code that addresses the problem I am having. The error message I am getting is
Function result 'sample' at (1) has no IMPLICIT type.
I label where line (1) is below.
I tried to follow this other question, however I wasn't able to figure it out. This function is within a module in my program and I made sure that the module has contains and I end the module after this function.
I also use implicit none in this function so I'm not sure why I get this message. How can I fix this error message?
Adding Real or Complex in front of function works, but I don't really get why. Shouldn't I only be able to use complex since the arrays are complex inside the function? Which is more suitable for my actual function? Both yield no compilation errors.
real function Sample(func) !this is line (1)
!complex function Sample(func)
implicit none
integer :: n,m
real :: x,y
complex, dimension(-9:9,-9:9), intent(in) :: func
complex, dimension(-9:9,-9:9) :: LocalF
LocalF = func
do n=-9,9
do m=-9,9
x = real(n)*0.2
y = real(m)*0.2
LocalF(n,m)= cmplx(z1(x,y),z2(x,y)) !assume z1,z2 are well defined
end do
end do
end function Sample
In Fortran every function has a result. If you like you can think of the result as a value returned by the function. Like every other value in a Fortran program a function result has a type, and a kind and a rank too.
By default the function result has the same name as the function itself, and its declaration is prepended to the function declaration. For example, here
integer function add(m,n)
integer, intent(in) :: a,b
add = a+b
end function
the function is called add and you can see (a) that the result is of type integer (and of default kind and scalar) and (b) that the result is formed by adding the two arguments together.
For functions returning arrays this syntax is not available, so you couldn't write something like
integer(1:4) add_vec(m1,m2)
In such cases you have to explicitly define the name (and later type and kind) of the result variable. Sticking with the simple example, something like
function add(m,n) result(addvec)
integer, intent(in) :: a(4),b(4)
integer, dimension(4) :: addvec
....
end function
Notice that you don't define the intent of the result.
In OP's case sample is, I think, intended to return a rank-2 array of complex values. I think OP needs to replace
function Sample(func) !this is line (1)
with
function Sample(func) result(LocalF)
and see how that goes. Here, if it is not evident already, you learn that the result name doesn't have to be the same as the name of the function.
Furthermore ... Adding Real or Complex in front of function works, but I don't really get why.
It might work in the sense of compiling, but executing it will lead to tears. By telling the compiler that the function result is either a real or complex value you satisfy the syntactical requirements for a function definition. But without assigning a (real or complex as declared) value to the result variable (called Sample in OP's code) the function will, at best, return junk.
To be as clear as I can ... in OP's original code there were two serious mistakes:
The function (result) was not given an explicit type, which lead to the compiler message shown.
The function did not include setting the value of the result variable, i.e. the variable with the same name as the function (in the absence of the result clause).
Procedures in Fortran come in two types: functions and subroutines. This question is about functions, so I'll consider just those.
What was missing in the first revision, giving the error about the implicit type of the function result1, was the result type.
Adding real function ... or complex function ..., etc., resolves that problem by explicitly giving the type of the function result. The linked documentation gives other ways of doing that.
The function's result is used when the function is referenced. When we have a reference like
func0 = Sample(func)
in the main program, the function Sample is invoked and the function result is defined in its execution. At the end of the function's execution its result is placed in the expression of the reference.
So, if you declare
real function Sample(func)
or
complex function Sample(func)
what you are saying is that the function result is either a real or complex entity. And when the function is evaluated, whatever value Sample had at the end is used in the expression (here assignment).
As a consequence of the function result being returned through Sample (in this case) we need to define its value. The important thing to note for the question, then, is that LocalF is a variable local to the function. If you mean it to be the result of the function you need to use the function result.
You have a number of options:
function Sample(func)
<type>, <attributes> :: sample ! Instead of LocalF
... :: func
end function
or
function Sample(func) result(LocalF)
<type>, <attributes> :: LocalF
... :: func
end function
You can even have
<type> function Sample(func)
<attribute statements for Sample>
... func
end function
but I really suggest you avoid that last one.
1 Note the error here is about type for the function result; in the linked question simply about the function when referenced.
Is there a way to pass a tuple as the parameters of a Lua function?
For example, I have a function that returns multiple values
function f(a,b) return b,a end
and I want this function f to be repeatedly applied, so I can write:
f (f ... f(1,2))
But what if I need to store this initial tuple (1,2) as a variable init?
f (f ... f(init))
Is there support for this?
According to this answer, it seems python has it with the splat operator *.
Lua does not have "tuples".
When a function returns multiple values, it returns multiple values. They aren't put together into some data structure; they're separate values. If you want to store multiple return values in a single data structure, you have to actually do that.
Lua 5.2 has the table.pack function, which you can use to store multiple return values in a table. But it does so in such a way that you can decompose them later:
local values = table.pack(f(1, 2))
f(table.unpack(values, values.n))
unpack exists in Lua 5.1, but pack does not. You can emulate it easily enough:
function pack(...)
return {n = select("#", ...), ...}
end
What is the difference between function and procedure ?Aprart from returning value
Because function can also be used as a procedure if you dont return any value then what is the difference...then what is the use of functions ?
Please specify a scenario where we can use functions and procedures ??
Since your title specifies VBA and VB6, I'll reference the type of subroutines used by those languages. VBA and VB6 use "Function" for subroutines that return a value and "Sub" for those that do not. It's certainly possible to use a Function for all of your subroutines and just ignore the return value. Unlike C++ and many other languages, you're not required to return a value from a VB function. Because VB automatically initializes all variables to default values (zero for number types, False for Boolean, the empty string for String, etc), any functions that don't explicitly return a value will simply return their default value, which you can ignore. For example:
Function MyFunc() As Long
' Nothing here
End Function
This function will return the value zero.
So while you can use Function in place of Sub and just ignore the return values, it's not a good programming practice. Other users of your code will assume you chose Function instead of Sub because you intended to return something meaningful and will likely be surprised to discover that you're not returning anything at all!
There MAY also be a slight performance hit when using Function vs Sub, due to the extra parameter value that is passed on the calling stack (the return value).