I have a method and I want to declare it with **parametric** and with **return type**.

What is the syntax?

```
f(a::T)::Bool where {T <: Number} = a == 1
```

The error message is:

```
ERROR: UndefVarError: T not defined
Stacktrace:
[1] top-level scope at REPL[4]:1
```

I know that there is another way to resolve this problem, but it’s a simplified version of my real problem. It is: I have a method declared with a return type and now I want to put a parametric argument.

The real question is: there is a syntax to declare both parametric argument and return type?

Thanks.

It’s just a slightly annoying parsing issue. Your definition is getting parsed as `f(a::T)::(Bool where {T <: Number})`

, which isn’t what you want. You can do:

```
julia> (f(a::T)::Bool) where {T <: Number} = a == 1
f (generic function with 1 method)
julia> f(1.0)
true
```

And by the way, you can see for yourself how the expression is parsed by quoting it, in case you’re ever curious:

```
julia> :(f(a::T)::Bool where {T <: Number} = a == 1)
:(f(a::T)::(Bool where T <: Number) = begin
#= REPL[3]:1 =#
a == 1
end)
```

note how the `(Bool where T <: Number)`

is grouped together.

5 Likes

Alternatively, you might find that the long-form function syntax is more readable here (and doesn’t have this parsing issue):

```
julia> function f(a::T)::Bool where {T <: Number}
a == 1
end
f (generic function with 1 method)
julia> f(1)
true
```

3 Likes