Problems with operators with "=" in them

[pao]

While trying to emulate Haskell syntax for the monad module I'm suddenly working on, I tried to define >>= as an alias for bind(f::Function, m::Monad). Julia lets me make the definition, but doesn't let me use it:

bind(f::Function, m::Monad) = join(fmap(f, m))
(>>=)(m::Monad, f::Function) = bind(f, m)

julia> ss = bind(get) do st 
           (value, newSt) = ("incr!", st+1)
           mthen(mreturn(State, value), put(newSt))
       end
State(#<function>)

julia> ss = get >>= st -> begin
           (value, newSt) = ("incr!", st+1)
           mthen(mreturn(State, value), put(newSt))
       end
no method >>(State,Function)
 in method_missing at base.jl:70

(entire WIP monad.jl: https://gist.github.com/3677645)

I get different errors if I try to simplify things:

julia> (>>=)(a, b) = a + b

julia> 1 >>= 2
syntax error: invalid assignment lvalue 1

julia> a = 1 >>= 2
syntax error: invalid assignment lvalue 1

Clearly the = in >>= is the source of confusion. Do we want to fix things so >>= can be legal infix, or ban = from infix operators?

[JeffBezanson]

It's just that a>>=b is converted to a=a>>b. You would have to call it using (>>=)(a,b).

[pao]

Which defeats the purpose in this case--the point would be to make it usable infix. So this is an extension on +=, -=, etc.? Didn't know that operators did that automatically. I'm inclined to think you shouldn't be allowed to define an operator ending in =, then, since the behavior won't be intuitive? Not that people are too likely to run into this...

[pao]

I can't define the fish operator >=> at all (syntax error). I'm coming to the opinion that = in operators is bad.

[JeffBezanson]

Very possible. The trouble is that >>= etc. exist as symbols in the parse tree, so you have to be able to write things like :(>>=). So I'm not sure at what point to raise the error.

[pao]

Can it be picked up when the method body is bound to the name (I assume this happens at some point), or is inspecting the string representation of the symbol not possible at that point?

[JeffBezanson]

That can be done, but then it maybe also has to be disallowed on the left of an assignment?

[pao]

Thanks! While we won't be able to implement a subset of Haskell with Julia macros, at least we'll be consistent about it!