improve List.indexedMap example

[stevensonmt]

The current example for List.indexedMap is

indexedMap (,) ["Tom","Sue","Bob"] == [ (0,"Tom"), (1,"Sue"), (2,"Bob") ]

The function being applied to the map is not immediately clear in this syntax. I would propose the following alternative example:

indexedMap ( \i n -> i * n ) [ 1, 2, 3, 4 ] == [ 0, 2, 6, 12 ]

Perhaps this could be followed by an example in which the index parameter is inferred, as in the current example.

[timbuckley]

Personally, I prefer the first example because the relationship between the index argument and the 2 in (2, "Bob") is more obvious than the relationship between 4 and 12 in your example, especially to those who are not quite as math-savvy.

Perhaps we can still improve on the clarity on the first by not using the (,) to-tuple operator by instead using a lambda function which does the same:

indexedMap (\i x -> (i, x)) ["Tom","Sue","Bob"] == [ (0,"Tom"), (1,"Sue"), (2,"Bob") ]

But there is another benefit to the original, which is that it teaches both the (,) operator (a useful function in itself) and indexedMap at the same time. Showing how different functions can be combined together is extremely useful for those learning Elm, IMO.

[stevensonmt]

That's a fair point. Perhaps using

indexedMap (\i x -> (i, x)) ["Tom","Sue","Bob"] == [ (0,"Tom"), (1,"Sue"), (2,"Bob") ]

and then pointing out that the anonymous function could be replaced by the (,) operator would be the best option.

[drathier]

Tuple.pair is what it should use.

[frozar]

I don't know for you but when I tried the example out of the box (elm 0.19), I get the following output:

> List.indexedMap (,) ["Tom","Sue","Bob"]                                  
-- PARSE ERROR ------------------------------------------------------------- elm

Something went wrong while parsing an expression (in parentheses) in
repl_value_1's definition.

4|   List.indexedMap (,) ["Tom","Sue","Bob"]
                      ^
I was expecting:

  - an expression, like x or 42
  - a closing paren
  - a minus sign (-)
  - an infix operator, like (+) or (==)

So for me, the example is just wrong (or I miss something). Anyway, thank a lot for right examples here that help to undersetand this feature.

[drathier]

Yeah, the (,) tuple constructor syntax was removed in 0.19.

[evancz]

The syntax changed to List.indexedMap Tuple.pair ["Tom","Sue","Bob"] in 0.19.0, but tracking in #1068 for the more explicit example.