nushell-usage

Nushell Usage Patterns

Critical Distinctions

Pipeline Input vs Parameters

CRITICAL: Pipeline input ($in) is NOT interchangeable with function parameters!

# ❌ WRONG - treats $in as first parameter
def my-func [list: list, value: any] {
    $list | append $value
}

# ✅ CORRECT - declares pipeline signature
def my-func [value: any]: list -> list {
    $in | append $value
}

# Usage
[1 2 3] | my-func 4  # Works correctly
my-func [1 2 3] 4    # ERROR! my-func doesn't take positional params

This applies to closures too.

Why this matters:

• Pipeline input can be lazily evaluated (streaming)
• Parameters are eagerly evaluated (loaded into memory)
• Different calling conventions entirely

Type Signatures

# No pipeline input
def func [x: int] { ... }                    # (x) -> output

# Pipeline input only
def func []: string -> int { ... }           # string | func -> int

# Both pipeline and parameters
def func [x: int]: string -> int { ... }     # string | func x -> int

# Generic pipeline
def func []: any -> any { ... }              # works with any input type

Common Patterns

Working with Lists

# Filter with index
$list | enumerate | where {|e| $e.index > 5 and $e.item.some-bool-field}

# Transform with previous state
$list | reduce --fold 0 {|item, acc| $acc + $item.value}

Working with Records

# Create record
{name: "Alice", age: 30}

# Merge records (right-biased)
$rec1 | merge $rec2

# Merge many records (right-biased)
[$rec1 $rec2 $rec3 $rec4] | into record

# Update field
$rec | update name {|r| $"Dr. ($r.name)"}

# Insert field
$rec | insert active true

# Insert field based on existing fields
{x:1, y: 2} | insert z {|r| $r.x + $r.y}

# Upsert (update or insert)
$rec | upsert count {|r| ($r.count? | default 0) + 1}

# Reject fields
$rec | reject password secret_key

# Select fields
$rec | select name age email

Working with Tables

# Tables are lists of records
let table = [
    {name: "Alice", age: 30}
    {name: "Bob", age: 25}
]

# Filter rows
$table | where age > 25

# Add column
$table | insert retired {|row| $row.age > 65}

# Rename column
$table | rename -c {age: years}

# Group by
$table | group-by status --to-table

# Transpose (rows ↔ columns)
$table | transpose name data