www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Cannot use local lambda as parameter to non-global template

reply =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
Why do I get errors like

template instance remove!((_) => _ == 1) cannot use local 
'__lambda5' as parameter to non-global template remove()(in K key)

for

     x.remove!(_ => _ == 1);

but not for

     x.remove!"a == 11";

?

How are these two lambdas different?

The function `remove` is a templated member of a struct instance 
`x`.
Jan 15
next sibling parent Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Monday, 15 January 2018 at 13:55:57 UTC, Nordlöw wrote:
 Why do I get errors like

 template instance remove!((_) => _ == 1) cannot use local 
 '__lambda5' as parameter to non-global template remove()(in K 
 key)

 for

     x.remove!(_ => _ == 1);

 but not for

     x.remove!"a == 11";

 ?

 How are these two lambdas different?

 The function `remove` is a templated member of a struct 
 instance `x`.
It's a bit involved, but boils down to this: lambdas in D can only have one context pointer. A simple example: class Foo { int n = 2; int run(alias fun)(int m) { return fun(n, m); } } unittest { int a = 3; int fn(int n, int m) { return a + n + m; } assert((new Foo).run!fn(4) == 9); } fn obviously requires a pointer to the context in the unittest block, since it uses 'a' in that context. Somewhat less obviously, it also needs a pointer to the Foo instance in which it is run. Since 2 > 1, this fails. There's a bug reported for it: https://issues.dlang.org/show_bug.cgi?id=5710, and even a $200 bounty for fixing it: https://www.bountysource.com/issues/1375082 The bug report is almost seven years old, so it seems this is a hard bug to squash. Luckily, there are some workarounds. One you've already found. Others include moving templates to scopes where less context is needed (e.g. moving a function template that is associated with a class, but doesn't need to be part of it, outside the class), and marking your lambdas 'static'. A static lambda does not have a context pointer to the scope in which it is declared, and so cannot use variables declared in it. For instance, fn above uses 'a' in the same scope, and so cannot be marked static. -- Simen
Jan 15
prev sibling next sibling parent Jacob Carlborg <doob me.com> writes:
On 2018-01-15 14:55, Nordlöw wrote:
 Why do I get errors like
 
 template instance remove!((_) => _ == 1) cannot use local '__lambda5' as 
 parameter to non-global template remove()(in K key)
 
 for
 
      x.remove!(_ => _ == 1);
 
 but not for
 
      x.remove!"a == 11";
 
 ?
 
 How are these two lambdas different?
 
 The function `remove` is a templated member of a struct instance `x`.
The difference is that the string version would not capture the surrounding context while the lambda version does. It's something related to that the lambda needs to capture both the this object, since "remove" is member of a struct, and capturing the local context of where "remove" is called. A lambda/delegate can only have one context pointer. There's an issue already reported for this. -- /Jacob Carlborg
Jan 15
prev sibling parent reply Meta <jared771 gmail.com> writes:
On Monday, 15 January 2018 at 13:55:57 UTC, Nordlöw wrote:
 Why do I get errors like

 template instance remove!((_) => _ == 1) cannot use local 
 '__lambda5' as parameter to non-global template remove()(in K 
 key)

 for

     x.remove!(_ => _ == 1);

 but not for

     x.remove!"a == 11";

 ?

 How are these two lambdas different?

 The function `remove` is a templated member of a struct 
 instance `x`.
Can you give a bit more context? The following code does not cause a compile error: import std.stdio; import std.algorithm; void main() { auto arr = [1, 2, 3]; arr.remove!(_ => _ == 1); writeln(arr); } Or was that code meant as an example?
Jan 15
next sibling parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 01/15/2018 07:27 AM, Meta wrote:
 On Monday, 15 January 2018 at 13:55:57 UTC, Nordlöw wrote:
 The function `remove` is a templated member of a struct instance `x`.
Can you give a bit more context? The following code does not cause a compile error: import std.stdio; import std.algorithm; void main() {     auto arr = [1, 2, 3];     arr.remove!(_ => _ == 1);     writeln(arr); } Or was that code meant as an example?
import std.algorithm; struct S { int[] elements; void remove(alias func)() { // ... } void remove(string s)() { foreach (a; elements) { const cond = mixin(s); if (cond) { // do remove } } } } void main() { auto s = S(); s.remove!"a == 11"(); // Fails compilation: // s.remove!(_ => _ == 1); } Ali
Jan 15
prev sibling parent =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Monday, 15 January 2018 at 15:27:23 UTC, Meta wrote:
 void main()
 {
     auto arr = [1, 2, 3];
     arr.remove!(_ => _ == 1);
     writeln(arr);
 }

 Or was that code meant as an example?
The problem occurs when the templated function is a member of the struct `arr`. I've moved the algorithm into a free function here https://github.com/nordlow/phobos-next/blob/master/src/hashmap_or_hashset.d#L1190 instead. I've changed the name to `removeAllMatching` for now.
Jan 15