Differences from other programming languages¶
The Miller programming language is intended to be straightforward and familiar, as well as not overly complex. It doesn't try to break new ground in terms of syntax; there are no classes or closures, and so on.
While the Principle of Least Surprise is often held to, nonetheless the following may be surprising.
No ++ or --¶
There is no ++
or --
operator. To increment
x
, use x = x+1
or x += 1
, and similarly for decrement.
Semicolons as delimiters¶
You don't need a semicolon to end expressions, only to separate them. This
was done intentionally from the very start of Miller: you should be able to do
simple things like mlr put '$z = $x * $y' myfile.dat
without needing a
semicolon.
Note that since you also don't need a semicolon before or after closing curly
braces (such as begin
/end
blocks, if
-statements, for
-loops, etc.) it's
easy to key in a few semicolon-free statements, and then to forget a
semicolon where one is needed . The parser tries to remind you about semicolons
whenever there's a chance a missing semicolon might be involved in a parse
error.
mlr --csv --from example.csv put -q ' begin { @count = 0 # No semicolon required -- before closing curly brace } $x=1 # No semicolon required -- at end of expression '
mlr --csv --from example.csv put -q ' begin { @count = 0 # No semicolon required -- before closing curly brace } $x=1 # Needs a semicolon after it $y=2 # No semicolon required -- at end of expression '
mlr: cannot parse DSL expression. Parse error on token "$y" at line 6 column 3. Please check for missing semicolon. Expected one of: $ ; > >> | ? || ^^ && ?? ??? =~ !=~ == != <=> >= < <= ^ & << >>> + - .+ .- * / // % .* ./ .// . ** [ [[ [[[
elif¶
Miller has elif
, not else if
or elsif
.
If-statement variable scoping¶
Miller is simple-minded about scoping local variables to blocks. If you have
if (something) { x = 1 } else { x = 2 }
then there are two x
variable, each confined only to their enclosing curly
braces; there is no hoisting out of the if
and else
blocks.
A suggestion is
var x if (something) { x = 1 } else { x = 2 }
Required curly braces¶
Bodies for all compound statements must be enclosed in curly braces, even if the body is a single statement:
mlr ... put 'if ($x == 1) $y = 2' # Syntax error
mlr ... put 'if ($x == 1) { $y = 2 }' # This is OK
No autoconvert to boolean¶
I.e. ints/strings/etc are neither "truthy" nor "falsy".
Boolean tests in if
/while
/for
/etc must always take a boolean expression:
if (1) {...}
results in the parse error
Miller: conditional expression did not evaluate to boolean.
,
Likewise if (x) {...}
, unless x
is a variable of boolean type.
Please use if (x != 0) {...}
, etc.
Integer-preserving arithmetic¶
As discussed on the arithmetic page the sum, difference, and product of two integers is again an integer, unless overflow occurs -- in which case Miller tries to convert to float in the least obtrusive way possible.
Likewise, while quotient and remainder are generally pythonic, the quotient and exponentiation of two integers is an integer when possible.
$ mlr repl -q
[mlr] 6/2 3 [mlr] typeof(6/2) int [mlr] 6/5 1.2 [mlr] typeof(6/5) float [mlr] typeof(7**8) int [mlr] typeof(7**80) float
Print adds spaces around multiple arguments¶
As seen in the previous example,
print
with multiple
comma-delimited arguments fills in intervening spaces for you. If you want to
avoid this, use the dot operator for string-concatenation instead.
mlr -n put -q ' end { print "[", "a", "b", "c", "]"; print "[" . "a" . "b" . "c" . "]"; } '
[ a b c ] [abc]
Similarly, a final newline is printed for you; use printn
to avoid this.
String literals with double quotes only¶
In some languages, like Ruby and Bash, string literals can be in single quotes or double quotes,
where single quotes suppress \n
converting to a newline character and double quotes allowing it:
'a\nb'
prints as the four characters a
, \
, n
, and b
on one line; "a\nb"
prints as an
a
on one line and a b
on another.
In others, like Python and JavaScript, string literals can be in single quotes or double quotes,
interchangeably -- so you can have "don't"
or 'the "right" thing'
as you wish.
In yet others, such as C/C++ and Java, string literals are in double auotes, like "abc"
,
while single quotes are for character literals like 'a'
or '\n'
. In these, if s
is a non-empty string,
then s[0]
is its first character.
In the Miller programming language:
- String literals are always in double quotes, like
"abc"
. - String-indexing/slicing always results in strings (even of length 1):
"abc"[1:1]
is the string"a"
, and there is no notion in the Miller programming language of a character type. - The single-quote character plays no role whatsoever in the grammar of the Miller programming language.
- Single quotes are reserved for wrapping expressions at the system command line. For example, in
mlr put '$message = "hello"' ...
, theput
verb gets the string$message = "hello"
; the shell has consumed the outer single quotes by the time the Miller parser receives it. - Things are a little different on Windows, where
"""
sequences are sometimes necessary: see the Miller on Windows page.
Absent-null¶
Miller has a somewhat novel flavor of null data called absent: if a record
has a field x
then $y=$x
creates a field y
, but if it doesn't then the assignment
is skipped. See the null-data page for more
information.
Maps¶
See the maps page.
Arrays, including 1-up array indices¶
Arrays and strings are indexed starting with 1, not 0. This is discussed in detail on the arrays page and the strings page.
mlr --csv --from data/short.csv cat
word,value apple,37 ball,28 cat,54
mlr --csv --from data/short.csv put -q ' @records[NR] = $*; end { for (i = 1; i <= NR; i += 1) { print "Record", i, "has word", @records[i]["word"]; } } '
Record 1 has word apple Record 2 has word ball Record 3 has word cat
Also, slices for arrays and strings are doubly inclusive: x[3:5]
gets you
elements 3, 4, and 5 of the array or string named x
.
See the arrays page for more about arrays; see the strings page for more about strings.
Two-variable for-loops¶
Miller has a key-value loop flavor: whether x
is a map or array, in for (k,v in x) { ... }
the k
will be bound to successive map keys (for maps) or 1-up array indices (for arrays), and the v
will be bound to successive map values.
Semantics for one-variable for-loops¶
Miller also has a single-variable loop flavor. If x
is a map then for (e in x) { ... }
binds e
to successive map keys (not values as in PHP). But if x
is an array then for e in x) { ... }
binds e
to successive array values (not indices).
JSON parse, stringify, decode, and encode¶
Miller has the verbs
json-parse
and
json-stringify
, and the DSL functions
json_parse
and
json_stringify
.
In some other lannguages these are called json_decode
and json_encode
.