[elm-discuss] Difficulty learning Elm

[Leroy Campbell]

Rather than decode to nested Dict types, you could create types that represent 
your domain better (assuming the underlying JSON is well-structured). You can 
also use `andThen` to decode based on some field in your objects.

-- 
You received this message because you are subscribed to the Google Groups "Elm 
Discuss" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to elm-discuss+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

Re: [elm-discuss] Difficulty learning Elm

[J. E. Marca]

Hi thanks.  I was missing the Result library entirely!

On Tuesday, September 27, 2016 at 12:04:14 AM UTC-7, Peter Damoc wrote:
>
> When you do something that can fail in some way, Elm gives you back a type 
> that has the failure covered so that you can treat it properly. 
> The two most frequently used types are 
> - *Result* where you either have Ok someType  OR  Err someFailureType 
> - *Maybe* where you either have Just someType OR Nothing 
>
> In both cases, if you are 100% sure that the operation cannot fail or you 
> don't want to cover the fail case, you can provide a default value and 
> unpack the type 
>
> Result.withDefault defaultValue someResult 
>
> or 
>
> Maybe.withDefault defaultValue someMaybe
>
>
> You can read more in the Error handling section of the guide:
> https://guide.elm-lang.org/error_handling/
>
>
>
> ...
>

-- 
You received this message because you are subscribed to the Google Groups "Elm 
Discuss" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to elm-discuss+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

[elm-discuss] Difficulty learning Elm

[J. E. Marca]

Hi all,

I have a moderately complicated JSON object.  A sample looks like:

``` 
{
  "118_192": {
"Cloverdale P": {
"sum_lane_miles": 0,
"sum_single_unit_mt": 0,
"sum_vmt": 57,
"sum_combination_mt": 0
  }
}
}
```

Thousands of such top level entries, with a variable number of secondary 
entries, and two different types of tertiary entries depending on the 
"type" of the second entry.  I'm debating between a simple nested dict 
approach, and a more complicated nested dict of two alternative object 
types, but I can't get past Go with the repl in order to test out my ideas.

First, the most simple case, I can decode this with nested Dictionaries

What I did in the repl is:

```

import Json.Decode as Json exposing (..)
import Dict exposing (..)

inputString = """{  "118_192": {"Cloverdale P": {  
"sum_lane_miles": 0,  "sum_single_unit_mt": 0,  "sum_vmt": 57,  
"sum_combination_mt": 0},"CO P": {  "sum_lane_miles": 0,  
"sum_single_unit_mt": 0,  "sum_vmt": 43,  "sum_combination_mt": 
0}  }}"""

gridVolumes = dict ( dict (dict float))
dataDict = decodeString gridVolumes inputString
```

But that last line is a wild guess.  What I want to do is get "something" I 
can use as a dictionary, but Elm is giving me two things...a Result object 
and the dictionary.

```
> dataDict
Ok (Dict.fromList [("118_192",Dict.fromList [("CO P",Dict.fromList 
[("sum_combination_mt",0),("sum_lane_miles",0),("sum_single_unit_mt",0),("sum_vmt",43)]),("Cloverdale
 
P",Dict.fromList 
[("sum_combination_mt",0),("sum_lane_miles",0),("sum_single_unit_mt",0),("sum_vmt",57)])])])
: Result.Result
String
(Dict.Dict String (Dict.Dict String (Dict.Dict String Float)))
> 
```

How do I strip off the "Result" in the repl so I can touch the dictionary 
and play with various types of Dict.get commands?

Obvious to me that I'm just not getting a fundamental concept of the 
language, but I can't find the right point of view so far in the docs I've 
read...for example, the JSON help page 
(https://guide.elm-lang.org/interop/json.html) never actually seems to 
*assign* the result of running a JSON parser to a value, and the JSON 
library documentation has fragements of code, but no complete worked 
examples one can reproduce in the repl.

Any advice or pointers to docs I should read or reread would be 
appreciated.  Or just a simple "you're doing X wrong do Y instead to get 
access to the Dict String (Dict String (Dict String Float))) thing" would 
be welcome too!

Regards,
James

-- 
You received this message because you are subscribed to the Google Groups "Elm 
Discuss" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to elm-discuss+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.