Here you go:
[https://play.nim-lang.org/#ix=26DP](https://play.nim-lang.org/#ix=26DP), just
one extra line and your code works.
The way this work is that templates are a way to insert premade AST into a
specified location, which means:
template cmpTaskField*(fieldName: untyped): untyped =
proc cmp(task1, task2: Task): int = cmp(task1.fieldName, task2.fieldName)
tasks = sorted(tasks, cmp = cmpTaskField(isComplete))
Run
generates:
tasks = sorted(tasks, cmp = proc cmp(task1, task2: Task): int =
cmp(task1.fieldName, task2.fieldName))
Run
and this is **not** valid Nim, because a named `proc` definition is a statement
rather than an expression.
But the anonymous proc syntax `proc () =` does not work inside templates, so
how can this be done? The answer is to define a proc then "return" its symbol:
template cmpTaskField*(fieldName: untyped): untyped =
proc cmp_custom(task1, task2: Task): int {.gensym.} =
cmp(task1.fieldName, task2.fieldName)
cmp_custom # <--- that's the proc (task1, task2: Task): int that we need
Run
The reason why I renamed `cmp` into `cmp_custom` is so that the compiler could
know exactly which `cmp` are we targeting.
