Let with pipeline

[sophiajt]

Description

This changes the default behaviour of let to be able to take a pipeline as its initial value.

For example:

> let x = "hello world" | str length

This is a change from the existing behaviour, where the right hand side is assumed to be an expression. Pipelines are more general, and can be more powerful.

My google foo is failing me, but this also fixes this issue:

let x = foo

Currently, this reads foo as a bareword that gets converted to a string rather than running the foo command. In practice, this is really annoying and is a really hard to spot bug in a script.

User-Facing Changes

BREAKING CHANGE BREAKING CHANGE

let gains the power to be assigned via a pipeline. However, this changes the behaviour of let x = foo from assigning the string "foo" to $x to being "run the command foo and give the result to $x"

Tests + Formatting

After Submitting

[hyiltiz]

So we can now let MYPY = /usr/bin/env python3 and then do let result = $MYPY hello.py?

[hyiltiz]

I tried let x = foo and it said it could not find the executable foo in PATH. If we are treating it as a command, shouldn't we only evaluate it at the call site rather than the name assigning site? As in, when I pointed the variable x to foo, the foo may not yet exist but I could call $x later in an environment where foo is well-defined. Maybe my expectation for the behavior is different here. I wonder if I am a special case for getting confused here.

[amtoine]

So we can now let MYPY = /usr/bin/env python3 and then do let result = $MYPY hello.py?

you should be able to do that yup 😊

I tried let x = foo and it said it could not find the executable foo in PATH. If we are treating it as a command, shouldn't we only evaluate it at the call site rather than the name assigning site? As in, when I pointed the variable x to foo, the foo may not yet exist but I could call $x later in an environment where foo is well-defined. Maybe my expectation for the behavior is different here. I wonder if I am a special case for getting confused here.

this might require some lazy evaluation of commands, right?
it's something we do not have now i think 🤔

[hyiltiz]

The lazy evaluation semantic is present in closures, i.e. let x = { foo } does not throw an error upon binding, but only during execution, i.e. when do $x is called. I am asking should the engine treat let x = foo with lazy evaluation semantics?

What is a pipeline? I am finding it confusing to think of pipelines as more general than expressions.