[Nix-dev] Using string as path to eg. builtins.readFile

[Roger Qiu]

Thanks, even if that doesn't go into the docs, I'm putting that into a 
github gist.


On 21/06/2016 5:44 PM, Bas van Dijk wrote:
> Hi Erik,
>
> Do note that I went through the same trouble as you figuring out how
> to apply builtins.readFile to a dynamically constructed path. Maybe we
> can document this "trick" in the documentation of builtins.readFile so
> other people don't have to go through the same trouble.
>
> To understand these things I would recommend using nix-repl:
>
>    $ nix-repl
>    Welcome to Nix version 1.11.2. Type :? for help.
>
>    nix-repl> name = "bar"
>
>    nix-repl> ./foo + "${name}"
>    /home/bas.van.dijk/foobar
>
> Note that the previous is equivalent to the simpler:
>
>    nix-repl> ./foo + name
>    /home/bas.van.dijk/foobar
>
> The reason that you get "foobar" is that we didn't include a "/". So
> lets try to do that:
>
>    nix-repl> ./foo/ + name
>    error: syntax error, unexpected '+', at (string):1:8
>
> Apparently the Nix path parser doesn't like a slash at the end of a
> path literal. So lets try adding the slash dynamically:
>
>    nix-repl> ./foo + "/" + name
>    /home/bas.van.dijk/foobar
>
> What happened here? Well, + is left associative so it is interpreted as:
>
>    (./foo + "/") + name
>
> Lets try evaluating that left expression alone:
>
>    nix-repl> ./foo + "/"
>    /home/bas.van.dijk/foo
>
> Apparently Nix performs normalization on paths since the final slash
> is not included. So lets put the parenthesis differently:
>
>    nix-repl> ./foo + ("/" + name)
>    /home/bas.van.dijk/foo/bar
>
> That's better! Now we can shorten this using some antiquotation:
>
>    nix-repl> ./foo + "/${name}"
>    /home/bas.van.dijk/foo/bar
>
> I hope this helped.
>
> Cheers,
>
> Bas
>
> On 21 June 2016 at 09:19, 4levels <4levels at gmail.com> wrote:
>> Hi Bas,
>>
>> Out of curiosity I've been experimenting with different approaches but only
>> yours works and I have no clue why..  Is there anything I can learn / read /
>> study to understand why?  Without having to study C++ (or python for that
>> matter as I can't even tell them apart ;-)
>> Is this basic lambda programming or very specific to Nix?
>>
>> Works
>> builtins.readFile (./keys + "/${name}")
>>
>> Doesn't work
>> builtins.readFile (./keys/ + "${name}") - unexpected +
>> builtins.readFile (./keys/ + name) - unexpected +
>> builtins.readFile (./keys + "/" + name") - no build errors, but doesn't
>> parse the / in the middle -> ./keysmancloud.amazon.iam.key_id -> not found
>>
>> Kind regards and thanks again, my key deployment sections have become very
>> compact and maintainable now..
>>
>> Erik
>>
>> On Tue, Jun 21, 2016 at 1:56 AM 4levels <4levels at gmail.com> wrote:
>>> Hi Bas,
>>>
>>> Thank you so much! Exactly what I was looking for.
>>> It's these basic things that I keep not getting my head around.. You have
>>> no idea how many fruitless Google and GitHub searches I tried on this one
>>> ;-)
>>>
>>> Finally resolved and another lesson learned, amazing!
>>>
>>> Kind regards,
>>>
>>> Erik
>>>
>>>
>>> On Tue, Jun 21, 2016, 01:39 Bas van Dijk <v.dijk.bas at gmail.com> wrote:
>>>> On 19 June 2016 at 15:58, 4levels <4levels at gmail.com> wrote:
>>>>> builtins.readFile "./keys/${name}"
>>>> Hi Erik, try this:
>>>>
>>>>    builtins.readFile (./keys + "/${name}")
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