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# DSL syntax

## Expression formatting

Multiple expressions may be given, separated by semicolons, and each may refer to the ones before:

<pre class="pre-highlight-in-pair">
<b>ruby -e '10.times{|i|puts "i=#{i}"}' | mlr --opprint put '$j = $i + 1; $k = $i +$j'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
i j  k
0 1  1
1 2  3
2 3  5
3 4  7
4 5  9
5 6  11
6 7  13
7 8  15
8 9  17
9 10 19
</pre>

Newlines within the expression are ignored, which can help increase legibility of complex expressions:

<pre class="pre-highlight-in-pair">
<b>mlr --opprint put '</b>
<b>  # Here is how to make a comment</b>
<b>  $nf       = NF;</b>
<b>  $nr       = NR;</b>
<b>  $fnr      = FNR;</b>
<b>  $filenum  = FILENUM;</b>
<b>  $filename = FILENAME</b>
<b>' data/small data/small2</b>
</pre>
<pre class="pre-non-highlight-in-pair">
a   b   i     x                    y                    nf nr fnr filenum filename
pan pan 1     0.346791             0.726802             5  1  1   1       data/small
eks pan 2     0.758679             0.522151             5  2  2   1       data/small
wye wye 3     0.204603             0.338318             5  3  3   1       data/small
eks wye 4     0.381399             0.134188             5  4  4   1       data/small
wye pan 5     0.573288             0.863624             5  5  5   1       data/small
pan eks 9999  0.267481232652199086 0.557077185510228001 5  6  1   2       data/small2
wye eks 10000 0.734806020620654365 0.884788571337605134 5  7  2   2       data/small2
pan wye 10001 0.870530722602517626 0.009854780514656930 5  8  3   2       data/small2
hat wye 10002 0.321507044286237609 0.568893318795083758 5  9  4   2       data/small2
pan zee 10003 0.272054845593895200 0.425789896597056627 5  10 5   2       data/small2
</pre>

Anything from a `#` character to the end of the line is a code comment.

<pre class="pre-highlight-in-pair">
<b>mlr --opprint filter '($x > 0.5 && $y < 0.5) || ($x < 0.5 && $y > 0.5)' \</b>
<b>  then stats2 -a corr -f x,y \</b>
<b>  data/medium</b>
</pre>
<pre class="pre-non-highlight-in-pair">
x_y_corr
-0.7479940285189345
</pre>

## Expressions from files

The simplest way to enter expressions for `put` and `filter` is between single quotes on the command line (see also [here](miller-on-windows.md) for Windows). For example:

<pre class="pre-highlight-in-pair">
<b>mlr --from data/small put '$xy = sqrt($x**2 + $y**2)'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
a=pan,b=pan,i=1,x=0.346791,y=0.726802,xy=0.805298171415408
a=eks,b=pan,i=2,x=0.758679,y=0.522151,xy=0.9209970096813562
a=wye,b=wye,i=3,x=0.204603,y=0.338318,xy=0.3953750836016352
a=eks,b=wye,i=4,x=0.381399,y=0.134188,xy=0.40431623334340655
a=wye,b=pan,i=5,x=0.573288,y=0.863624,xy=1.036583592538489
</pre>

<pre class="pre-highlight-in-pair">
<b>mlr --from data/small put 'func f(a, b) { return sqrt(a**2 + b**2) } $xy = f($x, $y)'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
a=pan,b=pan,i=1,x=0.346791,y=0.726802,xy=0.805298171415408
a=eks,b=pan,i=2,x=0.758679,y=0.522151,xy=0.9209970096813562
a=wye,b=wye,i=3,x=0.204603,y=0.338318,xy=0.3953750836016352
a=eks,b=wye,i=4,x=0.381399,y=0.134188,xy=0.40431623334340655
a=wye,b=pan,i=5,x=0.573288,y=0.863624,xy=1.036583592538489
</pre>

You may, though, find it convenient to put expressions into files for reuse, and read them
**using the -f option**. For example:

<pre class="pre-highlight-in-pair">
<b>cat data/fe-example-3.mlr</b>
</pre>
<pre class="pre-non-highlight-in-pair">
func f(a, b) {
  return sqrt(a**2 + b**2)
}
$xy = f($x, $y)
</pre>

<pre class="pre-highlight-in-pair">
<b>mlr --from data/small put -f data/fe-example-3.mlr</b>
</pre>
<pre class="pre-non-highlight-in-pair">
a=pan,b=pan,i=1,x=0.346791,y=0.726802,xy=0.805298171415408
a=eks,b=pan,i=2,x=0.758679,y=0.522151,xy=0.9209970096813562
a=wye,b=wye,i=3,x=0.204603,y=0.338318,xy=0.3953750836016352
a=eks,b=wye,i=4,x=0.381399,y=0.134188,xy=0.40431623334340655
a=wye,b=pan,i=5,x=0.573288,y=0.863624,xy=1.036583592538489
</pre>

If you have some of the logic in a file and you want to write the rest on the command line, you can **use the -f and -e options together**:

<pre class="pre-highlight-in-pair">
<b>cat data/fe-example-4.mlr</b>
</pre>
<pre class="pre-non-highlight-in-pair">
func f(a, b) {
  return sqrt(a**2 + b**2)
}
</pre>

<pre class="pre-highlight-in-pair">
<b>mlr --from data/small put -f data/fe-example-4.mlr -e '$xy = f($x, $y)'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
a=pan,b=pan,i=1,x=0.346791,y=0.726802,xy=0.805298171415408
a=eks,b=pan,i=2,x=0.758679,y=0.522151,xy=0.9209970096813562
a=wye,b=wye,i=3,x=0.204603,y=0.338318,xy=0.3953750836016352
a=eks,b=wye,i=4,x=0.381399,y=0.134188,xy=0.40431623334340655
a=wye,b=pan,i=5,x=0.573288,y=0.863624,xy=1.036583592538489
</pre>

A suggested use case here is defining functions in files and calling them from command-line expressions.

Another suggested use case is putting default parameter values in files, e.g., using `begin{@count=is_present(@count)?@count:10}` in the file, where you can precede that using `begin{@count=40}` using `-e`.

Moreover, you can have one or more `-f` expressions (maybe one function per file, for example) and one or more `-e` expressions on the command line.  If you mix `-f` and `-e`, then the expressions are evaluated in the order encountered.

## Semicolons, commas, newlines, and curly braces

Miller uses **semicolons as statement separators**, not statement terminators. This means you can write:

<pre class="pre-non-highlight-non-pair">
mlr put 'x=1'
mlr put 'x=1;$y=2'
mlr put 'x=1;$y=2;'
mlr put 'x=1;;;;$y=2;'
</pre>

Semicolons are optional after closing curly braces (which close conditionals and loops as discussed below).

<pre class="pre-highlight-in-pair">
<b>echo x=1,y=2 | mlr put 'while (NF < 10) { $[NF+1] = ""}  $foo = "bar"'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
x=1,y=2,3=,4=,5=,6=,7=,8=,9=,10=,foo=bar
</pre>

<pre class="pre-highlight-in-pair">
<b>echo x=1,y=2 | mlr put 'while (NF < 10) { $[NF+1] = ""}; $foo = "bar"'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
x=1,y=2,3=,4=,5=,6=,7=,8=,9=,10=,foo=bar
</pre>

Semicolons are required between statements, even if those statements are on separate lines.  **Newlines** are for your convenience but have no syntactic meaning: line endings do not terminate statements. For example, adjacent assignment statements must be separated by semicolons even if those statements are on separate lines:

<pre class="pre-non-highlight-non-pair">
mlr put '
  $x = 1
  $y = 2 # Syntax error
'

mlr put '
  $x = 1;
  $y = 2 # This is OK
'
</pre>

**Trailing commas** are allowed in function/subroutine definitions, function/subroutine callsites, and map literals. This is intended for (although not restricted to) the multi-line case:

<pre class="pre-highlight-in-pair">
<b>mlr --csvlite --from data/a.csv put '</b>
<b>  func f(</b>
<b>    num a,</b>
<b>    num b,</b>
<b>  ): num {</b>
<b>    return a**2 + b**2;</b>
<b>  }</b>
<b>  $* = {</b>
<b>    "s": $a + $b,</b>
<b>    "t": $a - $b,</b>
<b>    "u": f(</b>
<b>      $a,</b>
<b>      $b,</b>
<b>    ),</b>
<b>    "v": NR,</b>
<b>  }</b>
<b>'</b>
</pre>
<pre class="pre-non-highlight-in-pair">
s,t,u,v
3,-1,5,1
9,-1,41,2
</pre>

Bodies for all compound statements must be enclosed in **curly braces**, even if the body is a single statement:

<pre class="pre-highlight-non-pair">
<b>mlr put 'if ($x == 1) $y = 2' # Syntax error</b>
</pre>

<pre class="pre-highlight-non-pair">
<b>mlr put 'if ($x == 1) { $y = 2 }' # This is OK</b>
</pre>

Bodies for compound statements may be empty:

<pre class="pre-highlight-non-pair">
<b>mlr put 'if ($x == 1) { }' # This no-op is syntactically acceptable</b>
</pre>

