For Loop (for)

Laurence MorganAbout 3 minBuiltin CommandsLanguage And Scripting

For Loop (`for`)

A more familiar iteration loop to existing developers

Description

This for loop is fills a small niche where foreach or formap are inappropriate in your script. It's generally not recommended to use for because it performs slower and doesn't adhere to Murex's design philosophy. However it does offer additional flexibility around recursion.

Usage

for { variable; conditional; incrementor } { code-block } -> <stdout>

Examples

» for {$i=1; $i<6; $i++} { out "iteration $i" }
iteration 1
iteration 2
iteration 3
iteration 4
iteration 5

Detail

Syntax

for is a little naughty in terms of breaking Murex's style guidelines due to the first parameter being entered as one string treated as 3 separate code blocks. The syntax is like this for two reasons:

readability, having multiple { blocks } would make scripts unsightly
familiarity for those using to for loops in other languages

Take the following example:

for {$i=1; $i<6; $i++} { out "iteration $i" }

The first parameter is: {$i=1; $i<6; $i++}, this is then converted into the following code:

$i=1 - declare the loop iteration variable
$i<6 - if the condition is true then proceed to run the code in the second parameter - { echo $i }
$i++ - increment the loop iterator variable

The second parameter is the code to execute upon each iteration

Tips when writing JSON inside for loops

One of the drawbacks (or maybe advantages, depending on your perspective) of JSON is that parsers generally expect a complete file for processing in that the JSON specification requires closing tags for every opening tag. This means it's not always suitable for streaming. For example

» ja [1..3] -> foreach i { out ({ "$i": $i }) }
{ "1": 1 }
{ "2": 2 }
{ "3": 3 }

What does this even mean and how can you build a JSON file up sequentially?

One answer if to write the output in a streaming file format and convert back to JSON

» ja [1..3] -> foreach i { out (- "$i": $i) }
- "1": 1
- "2": 2
- "3": 3

» ja [1..3] -> foreach i { out (- "$i": $i) } -> cast yaml -> format json
[
    {
        "1": 1
    },
    {
        "2": 2
    },
    {
        "3": 3
    }
]

What if I'm returning an object rather than writing one?

The problem with building JSON structures from existing structures is that you can quickly end up with invalid JSON due to the specifications strict use of commas.

For example in the code below, each item block is it's own object and there are no [ ... ] encapsulating them to denote it is an array of objects, nor are the objects terminated by a comma.

» config -> [ shell ] -> formap k v { $v -> alter /Foo Bar }
{
    "Data-Type": "bool",
    "Default": true,
    "Description": "Display the interactive shell's hint text helper. Please note, even when this is disabled, it will still appear when used for regexp searches and other readline-specific functions",
    "Dynamic": false,
    "Foo": "Bar",
    "Global": true,
    "Value": true
}
{
    "Data-Type": "block",
    "Default": "{ progress $PID }",
    "Description": "Murex function to execute when an `exec` process is stopped",
    "Dynamic": false,
    "Foo": "Bar",
    "Global": true,
    "Value": "{ progress $PID }"
}
{
    "Data-Type": "bool",
    "Default": true,
    "Description": "ANSI escape sequences in Murex builtins to highlight syntax errors, history completions, {SGR} variables, etc",
    "Dynamic": false,
    "Foo": "Bar",
    "Global": true,
    "Value": true
}
...

Luckily JSON also has it's own streaming format: JSON lines (jsonl). We can cast this output as jsonl then format it back into valid JSON:

» config -> [ shell ] -> formap k v { $v -> alter /Foo Bar } -> cast jsonl -> format json
[
    {
        "Data-Type": "bool",
        "Default": true,
        "Description": "Write shell history (interactive shell) to disk",
        "Dynamic": false,
        "Foo": "Bar",
        "Global": true,
        "Value": true
    },
    {
        "Data-Type": "int",
        "Default": 4,
        "Description": "Maximum number of lines with auto-completion suggestions to display",
        "Dynamic": false,
        "Foo": "Bar",
        "Global": true,
        "Value": "6"
    },
    {
        "Data-Type": "bool",
        "Default": true,
        "Description": "Display some status information about the stop process when ctrl+z is pressed (conceptually similar to ctrl+t / SIGINFO on some BSDs)",
        "Dynamic": false,
        "Foo": "Bar",
        "Global": true,
        "Value": true
    },
...

`foreach` will automatically cast it's output as `jsonl` if it's stdin type is `json`

» ja [Tom,Dick,Sally] -> foreach name { out Hello $name }
Hello Tom
Hello Dick
Hello Sally

» ja [Tom,Dick,Sally] -> foreach name { out Hello $name } -> debug -> [[ /Data-Type/Murex ]]
jsonl

» ja [Tom,Dick,Sally] -> foreach name { out Hello $name } -> format json
[
    "Hello Tom",
    "Hello Dick",
    "Hello Sally"
]

For Loop (for)

# For Loop (for)

# Description

# Usage

# Examples

# Detail

# Syntax

# Tips when writing JSON inside for loops

# foreach will automatically cast it's output as jsonl if it's stdin type is json

# See Also