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digitalmars.D.learn - Recursive delegate inside template?

reply drathier <forum.dlang.org fi.fo> writes:
I'm trying to get this to compile, without much luck:

```
bool foo()
{
     template fn(A)
     {
         A delegate(A) fn;
         fn = delegate A(A a)
         {
             return fn(a);
         };
     }

     return fn!(bool)(true);
}
```

What am I missing here? How can I define a recursive delegate 
inside another function like this? I even got a gold linker error 
from run.dlang.io at one point: "TLS relocation against invalid 
instruction".

Removing the template works fine:

```
bool foo()
{
     alias A = bool;
     A delegate(A) fn;
     fn = delegate A(A a) { return fn(a); };

     return fn(true);
}
```
but I need the function to be polymorphic, so I need the template.
Jun 26 2020
next sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 26 June 2020 at 14:12:00 UTC, drathier wrote:

 I'm trying to get this to compile, without much luck:


You might be able to make it a function:


bool foo()
{
     auto fn(A)()
     {
         A delegate(A) fn;
         fn = delegate A(A a)
         {
             return fn(a);
         };
         return fn;
     }

     return fn!(bool)()(true);
}


 but I need the function to be polymorphic, so I need the 
 template.


I don't really understand your need though, can you post more 
context?
Jun 26 2020
parent drathier <forum.dlang.org fi.fo> writes:
On Friday, 26 June 2020 at 15:10:07 UTC, Adam D. Ruppe wrote:

 On Friday, 26 June 2020 at 14:12:00 UTC, drathier wrote:

 I'm trying to get this to compile, without much luck:


 You might be able to make it a function:


 bool foo()
 {
     auto fn(A)()
     {
         A delegate(A) fn;
         fn = delegate A(A a)
         {
             return fn(a);
         };
         return fn;
     }

     return fn!(bool)()(true);
 }


I tried this, but it gives me `Error: non-constant expression 
&fn` which I don't understand. Is ctfe kicking in even though 
this is inside a function?

```
void foo2()
{
     auto fn(A) = std.functional.toDelegate(({
             A fn(A)(A a)
             {
                 return a;
             }

             return &fn!(A);
         }()));
}
```


 but I need the function to be polymorphic, so I need the 
 template.


 I don't really understand your need though, can you post more 
 context?


I'm generating code, compiling a strict pure functional language 
to D. The code that I'm trying to compile is this example 
function:
```
fn a =
     let
         _ = fn a
     in a
```
which gets the type `forall a. a -> a` inferred.

That's something like
```
A delegate(A) fn(A) = delegate A (A a)
{
     auto _ = fn!(A)(a);
     return a;
};
```
in D syntax, if it would compile. Well, it compiles, but the 
linker fails `error: TLS relocation against invalid instruction`.

It's a simplified example that still tests these features:
- recursion
- polymorphism
- functions as values (closures/delegates)
Jun 26 2020
prev sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Jun 26, 2020 at 02:12:00PM +0000, drathier via Digitalmars-d-learn
wrote:

 I'm trying to get this to compile, without much luck:
 
 ```
 bool foo()
 {
     template fn(A)
     {
         A delegate(A) fn;
         fn = delegate A(A a)
         {
             return fn(a);
         };
     }
 
     return fn!(bool)(true);
 }
 ```
 
 What am I missing here? How can I define a recursive delegate inside
 another function like this?


[...]

Sounds like you're looking for the equivalent of the Y combinator,
perhaps?

	https://en.wikipedia.org/wiki/Fixed-point_combinator#Y_combinator


T

-- 
Never trust an operating system you don't have source for! -- Martin Schulze
Jun 26 2020