[elm-discuss] why does this code compile?

[Maris Orbidans]

https://gist.github.com/maruks/18fb2a23e1a2d019dc37563aa08ee46f



blur : Dict(Int,Int) Int -> ( Int, Int ) -> Int
blur cells ( x, y ) =
let
xs =
range (x - 1) (x + 1)

ys =
range (y - 1) (y + 1)

points =
concatMap (\x -> map (\y -> ( x, y )) ys) xs

*( sum, num )* =
foldr
(\p ( s, n ) ->
case Dict.get p cells of
Just h ->
* ( sum + h, num + 1 ) -- ( s + h, n + 1 )*

Nothing ->
( s, n )
)
( 0, 0 )
points
in
sum // num

[Aaron VonderHaar]

I'm assuming you're expecting an error because `sum` and `num` are values
that are referred to within their own definition?

The rule the compiler currently follows is that referring to yourself is
allowed if the reference is inside a lambda. (In this example, the
compiler does know whether or not `foldr` is going to immediately call the
lambda or leave it unevaluated.)

Post by Maris Orbidans
https://gist.github.com/maruks/18fb2a23e1a2d019dc37563aa08ee46f
blur : Dict(Int,Int) Int -> ( Int, Int ) -> Int
blur cells ( x, y ) =
let
xs =
range (x - 1) (x + 1)
ys =
range (y - 1) (y + 1)
points =
concatMap (\x -> map (\y -> ( x, y )) ys) xs
*( sum, num )* =
foldr
(\p ( s, n ) ->
case Dict.get p cells of
Just h ->
* ( sum + h, num + 1 ) -- ( s + h, n + 1 )*
Nothing ->
( s, n )
)
( 0, 0 )
points
in
sum // num
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[Gabriel Sartori]

Look at this simpler example, like Aaron said, the compiler does not
complain if the value is referred inside a lambda. But I think there is no
reason to allow this, or I am missing something?

https://ellie-app.com/kR68N5KWGa1/0

[Maris Orbidans]

It makes sense if something is lazy evaluated.

Post by Gabriel Sartori
Look at this simpler example, like Aaron said, the compiler does not
complain if the value is referred inside a lambda. But I think there is no
reason to allow this, or I am missing something?

https://ellie-app.com/kR68N5KWGa1/0