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 idioms will fail within your scripts. It’s generally not recommended to use for because it performs slower and doesn’t adhere to murex’s design philosophy.

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.

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.

» 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)

» 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

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Last built on Tue Sep 20 07:48:14 UTC 2022 against commit 1769d5b1769d5bb0ba4eaf204a5e0f52053b8f877bb7466.

Current version is 2.11.2030 which has been verified against 16798 tests cases.