How do functions work in Elixir? - function

In book "Programming Elixir" Dave Thomas shows an example
handle_open = fn
{:ok, file} -> "Read data: #{IO.read(file, :line)}"
{_, error} -> "Error: #{:file.format_error(error)}"
end
handle_open.(File.open("code/intro/hello.exs"))
handle_open.(File.open("nonexistent"))
I can't understand why second call of function goes into second flow.
Also why we don't pass an argument into func:
handle_open = fn(file)
?

This is a multi-clause anonymous function. handle_open has 2 clauses, one to match arguments with the pattern {:ok, file} and one with {_, error}. The first pattern that matches the arguments is executed. The given code is almost [1] equivalent to the following:
handle_open = fn arg ->
case arg do
{:ok, file} -> ...
{_, error} -> ...
end
end
So if the file exists, File.open will return {:ok, file} and the first clause will be executed. If it doesn't, {:error, error} will be returned which will match the second clause and execute that.
[1]: "Almost" because the error raised when none of the patterns match will be slightly different for the two cases.

This actually just mimics the pattern-match behaviour of Elixir.
Like you can have pattern match on your Module functions as well.
defmodule MyModule do
def read_file(path) do
File.open(path) |> handle_file()
end
defp handle_file({:ok, file}) do
// File there..
end
defp handle_file({_, error}) do
// Could not open file.. because of `error`
end
end
Like in the book example, I would try to open the file inside a case clause and delegate the outcome to the appropriate function:
defmodule FileReader do
def read_file(path) do
case File.open(path) do
{:ok, file} -> handle_file(file)
{_, error} -> handle_error(error)
end
end
defp handle_file(file) do
// File there..
end
defp handle_error(error) do
//
end
end

Related

Seeding the Database from a CSV file in Phoenix/Elixir

When I try to run: mix run priv/repo/seeds.exs, I have a problem: (FunctionClauseError) no function clause matching in anonymous fn/1 in :elixir_compiler_1.__FILE__/1 The following arguments were given to anonymous fn/1 in :elixir_compiler_1.__FILE__/1:
This is my seeds.exs file:
alias FlightsList.Repo
alias FlightsList.Management.Flights
File.stream!("C:/Users/vukap/phx_projects/flights_list/priv/repo/flights.csv")
|> Stream.drop(1)
|> CSV.decode(headers: [:Id, :Origin, :Destination, :DepartureDate, :DepartureTime, :ArrivalDate, :ArrivalTime, :Number])
|> Enum.each(fn {:ok, map} ->
Flights.changeset(
%Flights{},
%{Id: String.to_integer(map[:Id]), Origin: map[:Origin], Destination: map[:Destination], DepartureDate: String.to_integer(map[:DepartureDate]), DepartureTime: String.to_integer(map[:DepartureTime]), ArrivalDate: String.to_integer(map[:ArrivalDate]), ArrivalTime: String.to_integer(map[:ArrivalTime]), Number: map[:Number]})
|> Repo.insert!()
end)
How can I fix it?
It’s impossible to answer precisely until you have specified CSV library you used, or at least what the error actually says after The following arguments were given to anonymous fn/1, but the issue is definitely with CSV.decode/2 returning something different from {:ok, map} your next clause expects.
To fix this and similar issues, one would add the catch-all clause to the processing and examine the outcome.
...
|> Enum.each(fn
{:ok, map} -> Flights.changeset(...)
other -> IO.inspect(other, label: "Unexpected")
end)
Check what the above would print out and fix it accordingly.
I guess you are missing a separator in CSV.decode function, here is an example of how I do it, you can call stream_csv in seed file.
def store_it(row) do
{:ok, result} = row
%Segments{
id: result.id,
name: result.name
} |> Repo.insert!
end
def stream_csv do
Path.expand("~/Project/segmments.csv")
|> File.stream!
|> CSV.decode(separator: ?;, headers: [:id, :name])
|> Enum.each(&store_it/1)
end

Beginner in Elixir : parse CSV file

Hello I'm a beginner in Elixir and I want to parse and stock a CSV file in an Elixir object.
But it's display that:
** (FunctionClauseError) no function clause matching in anonymous fn/1 in Siren.parseCSV/0
The following arguments were given to anonymous fn/1 in Siren.parseCSV/0:
# 1
["41", "5", "59", "N", "80", "39", "0", "W", "Youngstown", "OH"]
anonymous fn/1 in Siren.parseCSV/0
(elixir 1.10.3) lib/stream.ex:482: anonymous fn/4 in Stream.filter/2
(elixir 1.10.3) lib/stream.ex:1449: Stream.do_element_resource/6
(elixir 1.10.3) lib/stream.ex:1609: Enumerable.Stream.do_each/4
(elixir 1.10.3) lib/enum.ex:959: Enum.find/3
(mix 1.10.3) lib/mix/task.ex:330: Mix.Task.run_task/3
(mix 1.10.3) lib/mix/cli.ex:82: Mix.CLI.run_task/2
Here my code:
defmodule Siren do
def parseCSV do
IO.puts("Let's parse CSV file...")
File.stream!("../name.csv")
|> Stream.map(&String.trim(&1))
|> Stream.map(&String.split(&1, ","))
|> Stream.filter(fn
["LatD" | _] -> false
end)
|> Enum.find(fn State -> String
[LatD, LatM, LatS, NS, LonD, LonM, LonS, EW, City, State] ->
IO.puts("find -> #{State}")
true
end)
end
end
And the csv file:
LatD,LatM,LatS,NS,LonD,LonM,LonS,EW,City,State
41,5,59,N,80,39,0,W,Youngstown,OH
42,52,48,N,97,23,23,W,Yankton,SD
46,35,59,N,120,30,36,W,Yakima,WA
42,16,12,N,71,48,0,W,Worcester,MA
43,37,48,N,89,46,11,W,WisconsinDells,WI
36,5,59,N,80,15,0,W,Winston-Salem,NC
49,52,48,N,97,9,0,W,Winnipeg,MB
39,11,23,N,78,9,36,W,Winchester,VA
34,14,24,N,77,55,11,W,Wilmington,NC
39,45,0,N,75,33,0,W,Wilmington,DE
48,9,0,N,103,37,12,W,Williston,ND
41,15,0,N,77,0,0,W,Williamsport,PA
37,40,48,N,82,16,47,W,Williamson,WV
33,54,0,N,98,29,23,W,WichitaFalls,TX
37,41,23,N,97,20,23,W,Wichita,KS
40,4,11,N,80,43,12,W,Wheeling,WV
26,43,11,N,80,3,0,W,WestPalmBeach,FL
47,25,11,N,120,19,11,W,Wenatchee,WA
41,25,11,N,122,23,23,W,Weed,CA
The first issue is here:
|> Stream.filter(fn
["LatD" | _] -> false
end)
all the lines should pass this and the only first one matches the given clauses. This would fix the issue
|> Stream.filter(fn
["LatD" | _] -> false
_ -> true
end)
or
|> Stream.reject(&match?(["LatD" | _], &1))
Enum.find(fn State -> String after looks unclear and would be surely the next issue. I failed to understand what have you tried to achieve here.
The general advice would be: don’t reinvent the wheel and use NimbleCSV written by José Valim to parse CSVs, because there are lot of corner cases (like commas inside quotes in any field etc,) handled properly in the aforementioned library.
Aleksei Matiushkin gave you the right answer but also you have this function:
fn
State ->
String
[LatD, LatM, LatS, NS, LonD, LonM, LonS, EW, City, State] ->
IO.puts("find -> #{State}")
true
end
It accepts two possible values, either State which is an atom, or a list of 10 specific atoms.
What you want to do is use variables, and variables in Elixir start with a lowercase letter or an underscore if it has to be ignored.
fn
state ->
String
[latd, latm, lats, ns, lond, lonm, lons, ew, city, state] ->
IO.puts("find -> #{state}")
true
end
But in this case, the first clause of the function will always match anything because it acts like a catch-all clause.
What you probably want is:
fn
[_latd, _latm, _lats, _ns, _lond, _lonm, _lons, _ew, _city, state] ->
IO.puts("find -> #{state}")
# here decide if you want to return true or false,
# for instance `state == NC`
true
end

In Elixir, How can I extract a lambda to a named function when the lambda is in a closure?

I have the following closure:
def get!(Item, id) do
Enum.find(
#items,
fn(item) -> item.id == id end
)
end
As I believe this looks ugly and difficult to read, I'd like to give this a name, like:
def get!(Item, id) do
defp has_target_id?(item), do: item.id = id
Enum.find(#items, has_target_id?/1)
end
Unfortunately, this results in:
== Compilation error in file lib/auction/fake_repo.ex ==
** (ArgumentError) cannot invoke defp/2 inside function/macro
(elixir) lib/kernel.ex:5238: Kernel.assert_no_function_scope/3
(elixir) lib/kernel.ex:4155: Kernel.define/4
(elixir) expanding macro: Kernel.defp/2
lib/auction/fake_repo.ex:28: Auction.FakeRepo.get!/2
Assuming it is possible, what is the correct way to do this?
The code you posted has an enormous amount of syntax errors/glitches. I would suggest you start with getting accustomed to the syntax, rather than trying to make Elixir better by inventing the things that nobody uses.
Here is the correct version that does what you wanted. The task might be accomplished with an anonymous function, although I hardly see a reason to make a perfectly looking idiomatic Elixir look ugly.
defmodule Foo do
#items [%{id: 1}, %{id: 2}, %{id: 3}]
def get!(id) do
has_target_id? = fn item -> item.id == id end
Enum.find(#items, has_target_id?)
end
end
Foo.get! 1
#⇒ %{id: 1}
Foo.get! 4
#⇒ nil
You can do this:
def get!(Item, id) do
Enum.find(
#items,
&compare_ids(&1, id)
)
end
defp compare_ids(%Item{}=item, id) do
item.id == id
end
But, that's equivalent to:
Enum.find(
#items,
fn item -> compare_ids(item, id) end
)
and may not pass your looks ugly and difficult to read test.
I was somehow under the impression Elixir supports nested functions?
Easy enough to test:
defmodule A do
def go do
def greet do
IO.puts "hello"
end
greet()
end
end
Same error:
$ iex a.ex
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
** (ArgumentError) cannot invoke def/2 inside function/macro
(elixir) lib/kernel.ex:5150: Kernel.assert_no_function_scope/3
(elixir) lib/kernel.ex:3906: Kernel.define/4
(elixir) expanding macro: Kernel.def/2
a.ex:3: A.go/0
wouldn't:
defp compare_ids(item, id), do: item.id == id
be enough? Is there any advantage to including %Item{} or making
separate functions for returning both true and false conditions?
What you gain by specifying the first parameter as:
func(%Item{} = item, target_id)
is that only an Item struct will match the first parameter. Here is an example:
defmodule Item do
defstruct [:id, :name, :description]
end
defmodule Dog do
defstruct [:id, :name, :owner]
end
defmodule A do
def go(%Item{} = item), do: IO.inspect(item.id, label: "id: ")
end
In iex:
iex(1)> item = %Item{id: 1, name: "book", description: "old"}
%Item{description: "old", id: 1, name: "book"}
iex(2)> dog = %Dog{id: 1, name: "fido", owner: "joe"}
%Dog{id: 1, name: "fido", owner: "joe"}
iex(3)> A.go item
id: : 1
1
iex(4)> A.go dog
** (FunctionClauseError) no function clause matching in A.go/1
The following arguments were given to A.go/1:
# 1
%Dog{id: 1, name: "fido", owner: "joe"}
a.ex:10: A.go/1
iex(4)>
You get a function clause error if you call the function with a non-Item, and the earlier an error occurs, the better, because it makes debugging easier.
Of course, by preventing the function from accepting other structs, you make the function less general--but because it's a private function, you can't call it from outside the module anyway. On the other hand, if you wanted to call the function on both Dog and Item structs, then you could simply specify the first parameter as:
|
V
func(%{}=thing, target_id)
then both an Item and a Dog would match--but not non-maps.
What you gain by specifying the first parameter as:
|
V
func(%Item{id: id}, target_id)
is that you let erlang's pattern matching engine extract the data you need, rather than calling item.id as you would need to do with this definition:
func(%Item{}=item, target_id)
In erlang, pattern matching in a parameter list is the most efficient/convenient/stylish way to write functions. You use pattern matching to extract the data that you want to use in the function body.
Going even further, if you write the function definition like this:
same variable name
| |
V V
func(%Item{id: target_id}, target_id)
then erlang's pattern matching engine not only extracts the value for the id field from the Item struct, but also checks that the value is equal to the value of the target_id variable in the 2nd argument.
Defining multiple function clauses is a common idiom in erlang, and it is considered good style because it takes advantage of pattern matching rather than logic inside the function body. Here's an erlang example:
get_evens(List) ->
get_evens(List, []).
get_evens([Head|Tail], Results) when Head rem 2 == 0 ->
get_evens(Tail, [Head|Results]);
get_evens([Head|Tail], Results) when Head rem 2 =/= 0 ->
get_evens(Tail, Results);
get_evens([], Results) ->
lists:reverse(Results).

No function clause matching in Regex.match?/2 on Elixir w/ Phoenix

I have a route in Phoenix that needs to check the JSON parameters with REGEX.
In my routine, I am creating a list of errors to report in the body in case one or more regexes fail.
But whenever I run the code, I get the "FunctionClauseError" error on "Regex.match?". I have tried String.match, but they evaluate to the same function.
Here is my code:
def postServidor(conn, parameters) do
reasons = []
error = False
if not Regex.match?(~r/^(19[0-9]{2}|2[0-9]{3})-(0[1-9]|1[012])-([123]0|[012][1-9]|31)$/, Map.get(parameters, "data_nascimento")) do
{error, reasons} = {True, reasons ++ [%{Reason => "[data_nascimento] missing or failed to match API requirements. It should look like this: 1969-02-12"}]}
end
if not Regex.match?(~r/^([A-Z][a-z]+([ ]?[a-z]?['-]?[A-Z][a-z]+)*)$/, Map.get(parameters, "nome")) do
{error, reasons} = {True, reasons ++ [%{Reason => "[name] missing or failed to match API requirements. It should look like this: Firstname Middlename(optional) Lastname"}]}
end
if not Regex.match?(~r/^([A-Z][a-z]+([ ]?[a-z]?['-]?[A-Z][a-z]+)*)$/, Map.get(parameters, "nome_identificacao")) do
{error, reasons} = {True, reasons ++ [%{Reason => "[nome_identificacao] missing or failed to match API requirements. It should look like this: Firstname Middlename(optional) Lastname"}]}
end
if not Regex.match?(~r/\b[MF]{1}\b/, Map.get(parameters, "sexo")) do
{error, reasons} = {True, reasons ++ [%{Reason => "[sexo] missing or failed to match API requirements. It should look like this: M for male, F for female"}]}
end
if not Regex.match?( ~r/\b[0-9]+\b/, Map.get(parameters, "id_pessoa")) do
{error, reasons} = {True, reasons ++ [%{Reason => "[id_pessoa] missing or failed to match API requirements. It should be numeric. "}]}
end
if not Regex.match?(~r/\b[0-9]+\b/, Map.get(parameters, "matricula_interna")) do
{error, reasons} = {True, reasons ++ [%{Reason => "[matricula_interna] missing or failed to match API requirements. It should be numeric. "}]}
end
if not Regex.match?(~r/\b[0-9]+\b/, Map.get(parameters, "siape")) do
{error, reasons} = {True, reasons ++ [%{Reason => "[siape] missing or failed to match API requirements. It should be numeric. "}]}
end
if error = True do
json put_status(conn, 400),reasons
else
IO.puts("ok")
end
end
Regex.match?/2 requires a string as its second argument. You're using Map.get/3 to potentially return that string, but if the key isn't found in the map, it defaults to nil, and that's my guess as to what's happening. If the key is not found in your map, it's passing nil to Regex.match?/2, for which there will be no function clause that matches. You can either fix your map to correctly have the key, or you can provide your own default string to use as a third parameter to Map.get/3. For example, Map.get(parameters, "data_nascimento", "some default").

What is the simplest way to do upsert with Ecto (MySQL)

Doing upsert is common in my app and I want to implement the cleanest and simple way to implement upsert.
Should I use fragments to implement native sql upsert?
Any idiomatic ecto way to do upsert?
You can use Ecto.Repo.insert_or_update/2, please note that for this to work, you will have to load existing models from the database.
model = %Post{id: 'existing_id', ...}
MyRepo.insert_or_update changeset
# => {:error, "id already exists"}
Example:
result =
case MyRepo.get(Post, id) do
nil -> %Post{id: id} # Post not found, we build one
post -> post # Post exists, using it
end
|> Post.changeset(changes)
|> MyRepo.insert_or_update
case result do
{:ok, model} -> # Inserted or updated with success
{:error, changeset} -> # Something went wrong
end
In my case insert_or_update raised an error due to the unique index constraint 🤔
What did work for me was Postgres v9.5 upsert through on_conflict parameter:
(considering unique column is called user_id)
changeset
|> MyRepo.insert(
on_conflict: :replace_all,
conflict_target: :user_id
)
If you're looking to upsert by something other than id, you can swap in get_by for get like this:
model = %User{email: "existing_or_new_email#heisenberg.net", name: "Cat", ...}
model |> User.upsert_by(:email)
# => {:found, %User{...}} || {:ok, %User{...}}
defmodule App.User do
alias App.{Repo, User}
def upsert_by(%User{} = record_struct, selector) do
case User |> Repo.get_by({selector, record_struct |> Map.get(selector)}) do
nil -> %User{} # build new user struct
user -> user # pass through existing user struct
end
|> User.changeset(record_struct |> Map.from_struct)
|> Repo.insert_or_update
end
end
On the off chance you're looking for a flexible approach that works across models and for multiple selectors (ie country + passport number), check out my hex package EctoConditionals!