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mers/README.md
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mers/README.md
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# mers
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Mers is a high-level programming language.
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It is designed to be safe (it doesn't crash at runtime) and as simple as possible.
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Install from *crates.io*:
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```sh
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cargo install mers
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```
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## what makes it special
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Mers is a simple, safe programming language.
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### Simplicity
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## features
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Mers is simple. There are only a few expressions:
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- mers' syntax is simple and concise
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- mers is type-checked, but behaves almost like a dynamically typed language
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- it has no nulls or exceptions
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- references in mers are explicit: `&var` vs. just `var`
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- no `goto`s (or `break`s or `return`s)
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- locking (useful for multithreading, any reference can be locked)
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- Values (`1`, `-2.4`, `"my string"`)
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- Blocks (`{ statement1, statement2 }` - comma is optional, newline preferred)
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- Tuples (`(5, 2)`) and Objects (`{ x: 5, y: 2 }`)
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- Variable initializations (`:=`)
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- Assignments (`=`)
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- Variables: `my_var`, `&my_var`
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- If statements: `if <condition> <then> [else <else>]`
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- Functions: `arg -> <do something>`
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- Function calls: `arg.function`
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+ or `arg1.function(arg2, arg3)` (nicer syntax for `(arg1, arg2, arg3).function`)
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- Type annotations: `[Int] (1, 2, 3).sum`
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+ Type definitions: `[[MyType] Int]`, `[[TypeOfMyVar] := my_var]`
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# examples
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Everything else is implemented as a function.
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## Hello, World!
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### Types and Safety
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Mers is built around a type-system where a value could be one of multiple types.
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In mers, `.function` is the syntax used to call functions.
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Everything before the `.` is the function's argument.
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In this case, our argument is the string containing *Hello, World!*,
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```
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x := if condition { 12 } else { "something went wrong" }
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```
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## Variables
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In mers, the compiler tracks all the types in your program,
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and it will catch every possible crash before the program even runs:
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If we tried to use `x` as an int, the compiler would complain since it might be a string, so this **does not compile**:
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```
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list := (1, 2, if true 3 else "not an int")
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list.sum.println
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```
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We use `name := value` to declare a variable, in this case `my_var`.
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We can then simply write `my_var` whenever we want to use its value.
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Type-safety for functions is different from what you might expect.
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You don't need to tell mers what type your function's argument has - you just use it however you want as if mers was a dynamically typed language:
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## If
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```
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sum_doubled := iter -> {
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one := iter.sum
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(one, one).sum
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An `if` is used to conditionally execute code.
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Obviously, since our condition is always `true`, our code will always run.
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The condition in an `if` has to be a bool, otherwise...
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## Else
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We can add `else` directly after an `if`. This is the code that will run if the condition was `false`.
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## Using If-Else to produce a value
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Depending on the languages you're used to, you may want to write something like this:
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```js
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var result
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if (condition) {
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result = "Yay"
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} else {
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result = "Nay"
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}
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(1, 2, 3).sum_doubled.println
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```
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We could try to use the function improperly by passing a string instead of an int:
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But in mers, an `if-else` can easily produce a value:
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```
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(1, 2, "3").sum_doubled.println
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We can shorten this even more by writing
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## What if the branches don't have the same type?
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Rust also allows us to return a value through `if-else` constructs, as long as they are of the same type:
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```rs
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if true {
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"Yep"
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} else {
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"Nay"
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}
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```
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But mers will catch this and show an error, because the call to `sum` inside of `sum_doubled` would fail.
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But as soon as we mix two different types, it no longer compiles:
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#### Type Annotations
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Eventually, mers' type-checking will cause a situation where calling `f1` causes an error,
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because it calls `f2`, which calls `f3`,
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which tries to call `f4` or `f5`, but neither of these calls are type-safe,
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because, within `f4`, ..., and within `f5`, ..., and so on.
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Error like this are basically unreadable, but they can happen.
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To prevent this, we should type-check our functions (at least the non-trivial ones) to make sure they do what we want them to do:
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```
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f1 := /* ... */
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// Calling `f1` with an int should cause it to return a string.
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// If `f1` can't be called with an int or it doesn't return a string, the compiler gives us an error here.
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[(Int -> String)] f1
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```rs
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if true {
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"Yep"
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} else {
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5 // Error!
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}
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```
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We can still try calling `f1` with non-int arguments, and it may still be type-safe and work perfectly fine.
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If you want to deny any non-int arguments, the type annotation has to be in `f1`'s declaration or you have to redeclare `f1`:
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In mers, this isn't an issue:
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```
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f1 := [(Int -> String)] /* ... */
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// or
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f1 := /* ... */
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f1 := [(Int -> String)] f1
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```
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This hard-limits the type of `f1`, similar to what you would expect from functions in statically typed programming languages.
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The variable `result` is simply assigned the type `String/Int`, so mers always knows that it has to be one of those two.
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However, even when using type annotations for functions, mers can be more dynamic than most other languages:
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We can see this if we add a type annotation:
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```
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f1 := [(Int/Float -> String, String -> ()/Float)] /* ... */
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```
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Here, `f1`'s return type depends on the argument's type: For numbers, `f1` returns a string, and for strings, `f1` returns an empty tuple or a float.
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Of course, if `f1`'s implementation doesn't satisfy these requirements, we get an error.
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Obviously, the `if-else` doesn't always return an `Int`, which is why we get an error.
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### Error Handling
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## Using If without Else to produce a value
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Errors in mers are normal values.
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For example, `("ls", ("/")).run_command` has the return type `({Int/Bool}, String, String)/RunCommandError`.
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This means it either returns the result of the command (exit code, stdout, stderr) or an error (a value of type `RunCommandError`).
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If there is no `else` branch, mers obviously has to show an error:
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So, if we want to print the programs stdout, we could try
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```
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(s, stdout, stderr) := ("ls", ("/")).run_command
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stdout.println
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```
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Or so you thought... But no, mers doesn't care. If the condition is false, it just falls back to an empty tuple `()`:
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But if we encountered a `RunCommandError`, mers wouldn't be able to assign the value to `(s, stdout, stderr)`, so this doesn't compile.
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Instead, we need to handle the error case, using the `try` function:
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```
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("ls", ("/")).run_command.try((
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(s, stdout, stderr) -> stdout.println,
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error -> error.println,
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))
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```
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## Sum of numbers
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For your own errors, you could use an object: `{err: { read_file_err: { path: /* ... */, reason: /* ... */ } } }`.
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This makes it clear that the value represents an error and it is convenient when pattern-matching:
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- `{ err: _ }`: all errors
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- `{ err: { read_file_err: _ } }`: only read-file errors
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- `{ err: { parse_err: _ } }`: only parse errors
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- `{ err: { read_file_err: { path: _, reason: { permission_denied: _ } } } }`: only read-file: permission-denied errors
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- ...
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## Sum of something else?
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If not all of the elements in our `numbers` tuple are actually numbers, this won't work.
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Instead, we'll get a type-error:
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## Loops
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This program asks the user for a number. if they type a valid number, it prints that number.
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If they don't type a valid number, they will be asked again.
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This works because `parse_float` returns `()/(Float)`, which happens to align with how loops in `mers` work:
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A `loop` will execute the code. If it is `()`, it will execute it again.
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If it is `(v)`, the loop stops and returns `v`:
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With this, we can loop forever:
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We can implement a while loop:
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Or a for loop:
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The `else (())` tells mers to exit the loop and return `()` once the condition returns `false`.
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## Functions
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Functions are expressed as `arg -> something`, where `arg` is the function's argument and `something` is what the function should do.
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It's usually convenient to assign the function to a variable so we can easily use it:
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Since functions are just normal values, we can pass them to other functions, and we can return them from other functions:
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Here, `do_twice` is a function which, given a function, returns a new function which executes the original function twice.
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So, `add_one.do_twice` becomes a new function which could have been written as `x -> x.add_one.add_one`.
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Of course, this doesn't compromise type-safety at all:
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Mers tells us that we can't call `add_two` with a `String`,
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because that would call the `func` defined in `do_twice` with that `String`, and that `func` is `add_one`,
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which would then call `sum` with that `String` and an `Int`, which doesn't work.
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The error may be a bit long, but it tells us what went wrong.
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We could make it a bit more obvious by adding some type annotations to our functions:
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## Advanced variables
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In mers, we can declare two variables with the same name:
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As long as the second variable is in scope, we can't access the first one anymore, because they have the same name.
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This is not the same as assigning a new value to x:
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The second `x` only exists inside the scope created by the code block (`{`), so, after it ends (`}`), `x` refers to the original variable again, whose value was not changed.
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To assign a new value to the original x, we have to write `&x =`:
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## References
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Writing `&var` returns a reference to `var`.
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We can then assign to that reference:
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... or:
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We aren't actually assigning to `ref` here, we are assigning to the variable to which `ref` is a reference.
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This works because the left side of an `=` doesn't have to be `&var`. As long as it returns a reference, we can assign to that reference:
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This is used, for example, by the `get_mut` function:
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Here, we pass a reference to our list (`&list`) and the index `0` to `get_mut`.
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`get_mut` then returns a `()/(&{Int/String})` - either nothing (if the index is out of bounds)
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or a reference to an element of the list, an `Int/String`. If it is a reference, we can assign a new value to it, which changes the list.
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## Multithreading
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(...)
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---
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Note: all of the pictures are screenshots of Alacritty after running `clear; mers pretty-print file main.mers && echo $'\e[1;35mOutput:\e[0m' && mers run file main.mers`.
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