Discussion:
why does this code compile?
(too old to reply)
Maris Orbidans
2017-11-26 20:54:21 UTC
Permalink
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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Aaron VonderHaar
2017-11-26 23:21:57 UTC
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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
2017-11-26 23:46:35 UTC
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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
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Maris Orbidans
2017-11-28 11:34:48 UTC
Permalink
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
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