murex Shell Docs

Command Reference: for

A more familiar iteration loop to existing developers

Description

This for loop is fills a small niche where foreach or formap are inappropiate 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; incrementation ) { code-block } -> <stdout>

Examples

» for ( i=1; i<6; i++ ) { echo $i }
1
2
3
4
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:

  1. readability (having multiple { blocks } would make scripts unsightly
  2. familiarity (for those using to for loops in other languages

The first parameter is: ( i=1; i<6; i++ ), but it is then converted into the following code:

  1. let i=0 - declare the loop iteration variable
  2. = i<0 - if the condition is true then proceed to run the code in the second parameter - { echo $i }
  3. let i++ - increment the loop iteration variable

The second parameter is the code to execute upon each iteration

Better for loops

Because each iteration of a for loop reruns the 2nd 2 parts in the first parameter (the conditional and incrementation), for is very slow. Plus the weird, non-idiomatic, way of writing the 3 parts, it’s fair to say for is not the recommended method of iteration and in fact there are better functions to achieve the same thing…most of the time at least.

For example:

a: [1..5] -> foreach: i { echo $i }
1
2
3
4
5

The different in performance can be measured. eg:

» time { a: [1..9999] -> foreach: i { out: <null> $i } }
0.097643108

» time { for ( i=1; i<10000; i=i+1 ) { out: <null> $i } }
0.663812496

You can also do step ranges with foreach:

» time { for ( i=10; i<10001; i=i+2 ) { out: <null> $i } }
0.346254973

» time { a: [1..999][0,2,4,6,8],10000 -> foreach i { out: <null> $i } }
0.053924326

…though granted the latter is a little less readable.

The big catch with using a piped into foreach is that values are passed as strings rather than numbers.

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"
]

See Also

This site's content is rebuilt automatically from murex's source code after each merge to the master branch. Downloadable murex binaries are also built with the website.

Last built on Tue Jan 31 12:58:18 UTC 2023 against commit c6bc4d8c6bc4d8f96c958dcb16d78e66d89468aef288078.

Current version is which has been verified against 13941 tests cases.