Runtime error with tuples

[TheSeamau5]

I am unable to pin down why this error happens but the failing code is surprisingly short:

smap : (a -> b -> (a,b)) -> a -> List b -> (a, List b)
smap f a list = case list of
  [] -> (a, [])
  b :: bs  ->
    let (a',b') = f a b
        (a'', list) = smap f a' bs
    in
        (a'', b' :: list)

test = smap (,) 0 [0]

which causes the following error:

Cannot read property '_0' of undefined


Open the developer console for more details.

[jvoigtlaender]

I think this is a compiler error that has to do with name shadowing. Note that the runtime error goes away if you change the code to this:

smap : (a -> b -> (a,b)) -> a -> List b -> (a, List b)
smap f a list = case list of
  [] -> (a, [])
  b :: bs  ->
    let (a',b') = f a b
        (a'', list') = smap f a' bs
    in
        (a'', b' :: list)

or to this:

smap : (a -> b -> (a,b)) -> a -> List b -> (a, List b)
smap f a list = case list of
  [] -> (a, [])
  b :: bs  ->
    let (a',b') = f a b
        (a'', list') = smap f a' bs
    in
        (a'', b' :: list')

Can you close this issue here, check whether a corresponding issue is already open at the elm-compiler repo, and if not, open an issue there?

[jvoigtlaender]

I think this is a compiler error that has to do with name shadowing. Note that the runtime error goes away if you change the code to this:

smap : (a -> b -> (a,b)) -> a -> List b -> (a, List b)
smap f a list = case list of
  [] -> (a, [])
  b :: bs  ->
    let (a',b') = f a b
        (a'', list') = smap f a' bs
    in
        (a'', b' :: list)

or to this:

smap : (a -> b -> (a,b)) -> a -> List b -> (a, List b)
smap f a list = case list of
  [] -> (a, [])
  b :: bs  ->
    let (a',b') = f a b
        (a'', list') = smap f a' bs
    in
        (a'', b' :: list')

Can you close this issue here, check whether a corresponding issue is already open at the elm-compiler repo, and if not, open an issue there?

[evancz]

When you reopen, do you mind also putting in the generated JS? I think that'd help a lot in figuring out exactly what went wrong.

[evancz]

When you reopen, do you mind also putting in the generated JS? I think that'd help a lot in figuring out exactly what went wrong.

[rtfeldman]

Cannot reproduce in 0.16.

I pasted this into try-elm:

import Html exposing (text)

main =
  text ("Hello, World!" ++ toString (test))

smap : (a -> b -> (a,b)) -> a -> List b -> (a, List b)
smap f a list = case list of
  [] -> (a, [])
  b :: bs  ->
    let (a',b') = f a b
        (a'', list) = smap f a' bs
    in
        (a'', b' :: list)

test = smap (,) 0 [0]

...and the screen displayed:

Hello, World!(0,[0])

@evancz I think this may have been fixed by the codegen updates in 0.16.

[rtfeldman]

Cannot reproduce in 0.16.

I pasted this into try-elm:

import Html exposing (text)

main =
  text ("Hello, World!" ++ toString (test))

smap : (a -> b -> (a,b)) -> a -> List b -> (a, List b)
smap f a list = case list of
  [] -> (a, [])
  b :: bs  ->
    let (a',b') = f a b
        (a'', list) = smap f a' bs
    in
        (a'', b' :: list)

test = smap (,) 0 [0]

...and the screen displayed:

Hello, World!(0,[0])

@evancz I think this may have been fixed by the codegen updates in 0.16.

[evancz]

Cool, thanks for following up on this!

[evancz]

Cool, thanks for following up on this!