Bug 1816 – Parameter names not visible in return type in function declarations

The following code should compile: typeof(a) identity(T)(T a) { return a; } void main() { auto x = identity(1); } In this case it's trivial to replace typeof(a) with T, but in the general case (e.g. mixins, complex expressions) it is necessary to access parameter names in complex expressions inside typeof().

[email protected] wrote: > http://d.puremagic.com/issues/show_bug.cgi?id=1816 > > Summary: Parameter names not visible in return type in function > declarations > Product: D > Version: unspecified > Platform: PC > OS/Version: Linux > Status: NEW > Severity: normal > Priority: P2 > Component: DMD > AssignedTo: [email protected] > ReportedBy: [email protected] > > > The following code should compile: > > typeof(a) identity(T)(T a) { return a; } > > void main() > { > auto x = identity(1); > } > > In this case it's trivial to replace typeof(a) with T, but in the general case > (e.g. mixins, complex expressions) it is necessary to access parameter names in > complex expressions inside typeof(). > > Here's a hack to work around that: either replace your parameter names with Init!(type) where Init == template Init(T) { T Init; } or use the return value of a delegate literal created on the spot. Example: typeof({ T a; return operationOnA(a); }) somefunc(T)(T a) { ... } I use that technique in scrapple.tools to say "if this string was our function body, what would its return type be?" It's fun! :D --downs

Thanks, downs, worked great. You forgot a pair of parens, so for others' reference here's the current implementation of binaryPred in std.functional: template binaryPred(string comp) { // @@@BUG@@@: typeof(mixin(comp)) should work typeof({ ElementType a, b; return mixin(comp);}()) binaryPred(ElementType)(ElementType a, ElementType b) { return mixin(comp); } } unittest { alias binaryPred!(q{a < b}) less; assert(less(1, 2) && !less(2, 1)); assert(less("1", "2") && !less("2", "1")); }

While the issue described here still exists, the modern-day equivalent of the give code would use auto, and any complex typeof() expressions would almost certainly need to be repeated inside the function body in any case, where the parameter is available. Closing this. If there are compelling use cases, reopen it with example code.