`echo` Throws Error When Used At Start Of Pipeline

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Understanding the Issue

When you have an echo as the first term of a pipeline in Gleam, the compiler returns a rather cryptic error. This issue can be frustrating, especially when you're trying to debug your code. In this article, we'll explore the reasons behind this error and provide a solution to help you overcome it.

What is a Pipeline in Gleam?

In Gleam, a pipeline is a sequence of expressions that are executed in a specific order. Pipelines are used to perform operations on data, such as filtering, mapping, and reducing. A pipeline typically starts with a function call, followed by a series of expressions that are executed in a specific order.

The Problem with echo at the Start of a Pipeline

The issue with using echo at the start of a pipeline is that it's not a function that returns a value. Instead, echo is a function that prints its arguments to the console. When you use echo at the start of a pipeline, the compiler gets confused because it's expecting a function that returns a value, but echo doesn't return anything.

The Error Message

The error message you see when you use echo at the start of a pipeline is:

{compiling,"/home/pedrohcf/dev/chess_bot/build/dev/erlang/chess_bot/_gleam_artefacts/tests.erl"}
src/tests.gleam:4:5: function echo/4 undefined
%    4| }
%     |  ^^^^

src/tests.gleam:5:14: variable '_pipe' is unbound
src/tests.gleam:4:10: Warning: variable '_pipe' is bound multiple times in this pattern.
If you mean to ignore this value, use '_' or
a different underscore-prefixed name
%    4| }
%     |  ^^^^^^^^^

{failed,"/home/pedrohcf/dev/chess_bot/build/dev/erlang/chess_bot/_gleam_artefacts/tests.erl"}
{compiled,chess_bot,
          "/home/pedrohcf/dev/chess_bot/build/dev/erlang/chess_bot/ebin/chess_bot.beam"}
ERROR Failed error=ShellCommand { program: "escript", reason: Unknown }
error: Shell command failure

There was a problem when running the shell command `escript`.

The Solution

To fix this issue, you need to use a function that returns a value at the start of the pipeline. One way to do this is to use the fn keyword to define a function that returns a value. Here's an example:

fn() {
  echo("Hello, World!")
}

In this example, we define a function fn that returns a value using the echo function. We can then use this function at the start of the pipeline:

fn() {
  echo("Hello, World!")
} | ...

By using a function that returns a value at the start of the pipeline, we can avoid the error message and write more efficient code.

Conclusion

In conclusion, using echo at the start of a pipeline in Gleam can cause a cryptic error message. To fix this issue, we need to use a function that returns a value at the start of the pipeline. By defining a function using the fn keyword, we can avoid the error message and write more efficient code. We hope this article has helped you understand the issue and provided a solution to help you overcome it.

Gleam Version

  • Gleam: 1.9.0

OS

  • Ubuntu 22.04.5 LTS

Runtime

  • Erlang

Logs

{compiling,"/home/pedrohcf/dev/chess_bot/build/dev/erlang/chess_bot/_gleam_artefacts/tests.erl"}
src/tests.gleam:4:5: function echo/4 undefined
%    4| }
%     |  ^^^^

src/tests.gleam:5:14: variable '_pipe' is unbound
src/tests.gleam:4:10: Warning: variable '_pipe' is bound multiple times in this pattern.
If you mean to ignore this value, use '_' or
a different underscore-prefixed name
%    4| }
%     |  ^^^^^^^^^

{failed,"/home/pedrohcf/dev/chess_bot/build/dev/erlang/chess_bot/_gleam_artefacts/tests.erl"}
{compiled,chess_bot,
          "/home/pedrohcf/dev/chess_bot/build/dev/erlang/chess_bot/ebin/chess_bot.beam"}
ERROR Failed error=ShellCommand { program: "escript", reason: Unknown }
error: Shell command failure

There was a problem when running the shell command `escript`.
```<br/>
**echo throws error when used at start of pipeline: Q&A**
=====================================================

**Q: What is the issue with using echo at the start of a pipeline in Gleam?**
-------------------------------------------------------------------

A: The issue with using `echo` at the start of a pipeline in Gleam is that it's not a function that returns a value. Instead, `echo` is a function that prints its arguments to the console. When you use `echo` at the start of a pipeline, the compiler gets confused because it's expecting a function that returns a value, but `echo` doesn't return anything.

**Q: What is a pipeline in Gleam?**
-------------------------------

A: In Gleam, a pipeline is a sequence of expressions that are executed in a specific order. Pipelines are used to perform operations on data, such as filtering, mapping, and reducing. A pipeline typically starts with a function call, followed by a series of expressions that are executed in a specific order.

**Q: How do I fix the error when using echo at the start of a pipeline?**
-------------------------------------------------------------------

A: To fix the error, you need to use a function that returns a value at the start of the pipeline. One way to do this is to use the `fn` keyword to define a function that returns a value. Here's an example:
```gleam
fn() {
  echo("Hello, World!")
}

In this example, we define a function fn that returns a value using the echo function. We can then use this function at the start of the pipeline:

fn() {
  echo("Hello, World!")
} | ...

By using a function that returns a value at the start of the pipeline, we can avoid the error message and write more efficient code.

Q: What is the difference between echo and a function that returns a value?

A: The main difference between echo and a function that returns a value is that echo prints its arguments to the console, while a function that returns a value returns a value that can be used in the pipeline.

Q: Can I use echo in a pipeline if I don't care about the output?

A: Yes, you can use echo in a pipeline if you don't care about the output. However, you need to use the _ variable to ignore the output. Here's an example:

echo("Hello, World!") | _

In this example, we use the _ variable to ignore the output of the echo function.

Q: What are some common use cases for pipelines in Gleam?

A: Some common use cases for pipelines in Gleam include:

  • Filtering data: You can use a pipeline to filter data based on certain conditions.
  • Mapping data: You can use a pipeline to transform data from one format to another.
  • Reducing data: You can use a pipeline to reduce data to a single value.

Q: How do I debug a pipeline in Gleam?

A: To debug a pipeline in Gleam, you can use the debug function to print the values of variables at different points in the pipeline. Here's an example:

fn() {
  x = 5
  debug(x)
  y = x * 2
  debug(y)
  echo(y)
}

In this example, we use the debug function to print the values of x and y at different points in the pipeline.

Q: What are some best practices for writing pipelines in Gleam?

A: Some best practices for writing pipelines in Gleam include:

  • Using functions that return values at the start of the pipeline.
  • Using the _ variable to ignore output when necessary.
  • Using the debug function to debug pipelines.
  • Keeping pipelines concise and easy to read.

By following these best practices, you can write efficient and effective pipelines in Gleam.