Module pl.utils

Generally useful routines.

See the Guide.

Dependencies: pl.compat, all exported fields and functions from pl.compat are also available in this module.

Functions

pack (...) pack an argument list into a table.
unpack (t[, i[, j]]) unpack a table and return its contents.
printf (fmt, ...) print an arbitrary number of arguments using a format.
fprintf (f, fmt, ...) write an arbitrary number of arguments to a file using a format.
import (t, T) take a table and 'inject' it into the local namespace.
choose (cond, value1, value2) return either of two values, depending on a condition.
array_tostring (t[, temp[, tostr]]) convert an array of values to strings.
is_type (obj, tp) is the object of the specified type?
npairs (t[, i_start=1[, i_end=t.n or #t[, step=1]]]) an iterator with indices, similar to ipairs, but with a range.

Tables

patterns Some standard patterns
stdmt Standard meta-tables as used by other Penlight modules

Error handling

assert_arg (n, val, tp, verify, msg, lev) assert that the given argument is in fact of the correct type.
enum (...) creates an Enum table.
function_arg (idx, f, msg) process a function argument.
assert_string (n, val) assert the common case that the argument is a string.
on_error (mode) control the error strategy used by Penlight.
raise (err) used by Penlight functions to return errors.

File handling

readfile (filename, is_bin) return the contents of a file as a string
writefile (filename, str, is_bin) write a string to a file
readlines (filename) return the contents of a file as a list of lines

OS functions

executeex (cmd, bin) execute a shell command and return the output.
quote_arg (argument) Quote and escape an argument of a command.
quit ([code], msg, ...) error out of this program gracefully.

String functions

escape (s) escape any Lua 'magic' characters in a string
split (s, re, plain, n) split a string into a list of strings separated by a delimiter.
splitv (s, re, plain, n) split a string into a number of return values.

Functional

memoize (func) 'memoize' a function (cache returned value for next call).
add_function_factory (mt, fun) associate a function factory with a type.
string_lambda (lf) an anonymous function as a string.
bind1 (fn, p) bind the first argument of the function to a value.
bind2 (fn, p) bind the second argument of the function to a value.

Deprecation

set_deprecation_func (func) Sets a deprecation warning function.
raise_deprecation (opts) raises a deprecation warning.


Functions

pack (...)
pack an argument list into a table.

Parameters:

  • ... any arguments

Returns:

    a table with field n set to the length

See also:

unpack (t[, i[, j]])
unpack a table and return its contents.

NOTE: this implementation differs from the Lua implementation in the way that this one DOES honor the n field in the table t, such that it is 'nil-safe'.

Parameters:

  • t table to unpack
  • i index from which to start unpacking, defaults to 1 (optional)
  • j index of the last element to unpack, defaults to t.n or else #t (optional)

Returns:

    multiple return values from the table

See also:

Usage:

    local t = table.pack(nil, nil, nil, 4)
    local a, b, c, d = table.unpack(t)   -- this unpack is NOT nil-safe, so d == nil
    
    local a, b, c, d = utils.unpack(t)   -- this is nil-safe, so d == 4
printf (fmt, ...)
print an arbitrary number of arguments using a format. Output will be sent to stdout.

Parameters:

  • fmt The format (see string.format)
  • ... Extra arguments for format
fprintf (f, fmt, ...)
write an arbitrary number of arguments to a file using a format.

Parameters:

  • f File handle to write to.
  • fmt The format (see string.format).
  • ... Extra arguments for format
import (t, T)
take a table and 'inject' it into the local namespace.

Parameters:

  • t The table (table), or module name (string), defaults to this utils module table
  • T An optional destination table (defaults to callers environment)
choose (cond, value1, value2)
return either of two values, depending on a condition.

Parameters:

  • cond A condition
  • value1 Value returned if cond is truthy
  • value2 Value returned if cond is falsy
array_tostring (t[, temp[, tostr]])
convert an array of values to strings.

Parameters:

  • t a list-like table
  • temp (table) buffer to use, otherwise allocate (optional)
  • tostr custom tostring function, called with (value,index). Defaults to tostring. (optional)

Returns:

    the converted buffer
is_type (obj, tp)
is the object of the specified type? If the type is a string, then use type, otherwise compare with metatable

Parameters:

  • obj An object to check
  • tp String of what type it should be

Returns:

    boolean

Usage:

    utils.is_type("hello world", "string")   --> true
    -- or check metatable
    local my_mt = {}
    local my_obj = setmetatable(my_obj, my_mt)
    utils.is_type(my_obj, my_mt)  --> true
npairs (t[, i_start=1[, i_end=t.n or #t[, step=1]]])
an iterator with indices, similar to ipairs, but with a range. This is a nil-safe index based iterator that will return nil when there is a hole in a list. To be safe ensure that table t.n contains the length.

Parameters:

  • t table the table to iterate over
  • i_start integer start index (default 1)
  • i_end integer end index (default t.n or #t)
  • step integer step size (default 1)

Returns:

  1. integer index
  2. any value at index (which can be nil!)

See also:

Usage:

    local t = utils.pack(nil, 123, nil)  -- adds an n field when packing
    
    for i, v in utils.npairs(t, 2) do  -- start at index 2
      t[i] = tostring(t[i])
    end
    
    -- t = { n = 3, [2] = "123", [3] = "nil" }

Tables

patterns
Some standard patterns

Fields:

  • FLOAT floating point number
  • INTEGER integer number
  • IDEN identifier
  • FILE file
stdmt
Standard meta-tables as used by other Penlight modules

Fields:

  • List the List metatable
  • Map the Map metatable
  • Set the Set metatable
  • MultiMap the MultiMap metatable

Error handling

assert_arg (n, val, tp, verify, msg, lev)
assert that the given argument is in fact of the correct type.

Parameters:

  • n argument index
  • val the value
  • tp the type
  • verify an optional verification function
  • msg an optional custom message
  • lev optional stack position for trace, default 2

Returns:

    the validated value

Raises:

if val is not the correct type

Usage:

    local param1 = assert_arg(1,"hello",'table')  --> error: argument 1 expected a 'table', got a 'string'
    local param4 = assert_arg(4,'!@#$%^&*','string',path.isdir,'not a directory')
         --> error: argument 4: '!@#$%^&*' not a directory
enum (...)
creates an Enum table. This helps prevent magic strings in code by throwing errors for accessing non-existing values.

Calling on the object does the same, but returns a soft error; nil + err.

The values are equal to the keys. The enum object is read-only.

Parameters:

  • ... strings that make up the enumeration.

Returns:

    Enum object

Usage:

  • -- accessing at runtime
    local obj = {}
    obj.MOVEMENT = utils.enum("FORWARD", "REVERSE", "LEFT", "RIGHT")
    
    if current_movement == obj.MOVEMENT.FORWARD then
      -- do something
    
    elseif current_movement == obj.MOVEMENT.REVERES then
      -- throws error due to typo 'REVERES', so a silent mistake becomes a hard error
      -- "'REVERES' is not a valid value (expected one of: 'FORWARD', 'REVERSE', 'LEFT', 'RIGHT')"
    
    end
  • -- validating user-input
    local parameter = "...some user provided option..."
    local ok, err = obj.MOVEMENT(parameter) -- calling on the object
    if not ok then
      print("bad 'parameter', " .. err)
      os.exit(1)
    end
function_arg (idx, f, msg)
process a function argument. This is used throughout Penlight and defines what is meant by a function: Something that is callable, or an operator string as defined by pl.operator, such as '>' or '#'. If a function factory has been registered for the type, it will be called to get the function.

Parameters:

  • idx argument index
  • f a function, operator string, or callable object
  • msg optional error message

Returns:

    a callable

Raises:

if idx is not a number or if f is not callable
assert_string (n, val)
assert the common case that the argument is a string.

Parameters:

  • n argument index
  • val a value that must be a string

Returns:

    the validated value

Raises:

val must be a string

Usage:

    local val = 42
    local param2 = utils.assert_string(2, val) --> error: argument 2 expected a 'string', got a 'number'
on_error (mode)

control the error strategy used by Penlight. This is a global setting that controls how utils.raise behaves:

  • 'default': return nil + error (this is the default)
  • 'error': throw a Lua error
  • 'quit': exit the program

Parameters:

  • mode either 'default', 'quit' or 'error'

See also:

raise (err)
used by Penlight functions to return errors. Its global behaviour is controlled by utils.on_error. To use this function you MUST use it in conjunction with return, since it might return nil + error.

Parameters:

  • err the error string.

See also:

Usage:

    if some_condition then
      return utils.raise("some condition was not met")  -- MUST use 'return'!
    end

File handling

readfile (filename, is_bin)
return the contents of a file as a string

Parameters:

  • filename The file path
  • is_bin open in binary mode

Returns:

    file contents
writefile (filename, str, is_bin)
write a string to a file

Parameters:

  • filename The file path
  • str The string
  • is_bin open in binary mode

Returns:

  1. true or nil
  2. error message

Raises:

error if filename or str aren't strings
readlines (filename)
return the contents of a file as a list of lines

Parameters:

  • filename The file path

Returns:

    file contents as a table

Raises:

error if filename is not a string

OS functions

executeex (cmd, bin)
execute a shell command and return the output. This function redirects the output to tempfiles and returns the content of those files.

Parameters:

  • cmd a shell command
  • bin boolean, if true, read output as binary file

Returns:

  1. true if successful
  2. actual return code
  3. stdout output (string)
  4. errout output (string)
quote_arg (argument)
Quote and escape an argument of a command. Quotes a single (or list of) argument(s) of a command to be passed to os.execute, pl.utils.execute or pl.utils.executeex.

Parameters:

  • argument (string or table/list) the argument to quote. If a list then all arguments in the list will be returned as a single string quoted.

Returns:

    quoted and escaped argument.

Usage:

    local options = utils.quote_arg {
        "-lluacov",
        "-e",
        "utils = print(require('pl.utils')._VERSION",
    }
    -- returns: -lluacov -e 'utils = print(require('\''pl.utils'\'')._VERSION'
quit ([code], msg, ...)
error out of this program gracefully.

Parameters:

  • code The exit code, defaults to -1 if omitted (optional)
  • msg The exit message will be sent to stderr (will be formatted with the extra parameters)
  • ... extra arguments for message's format'

See also:

Usage:

    utils.quit(-1, "Error '%s' happened", "42")
    -- is equivalent to
    utils.quit("Error '%s' happened", "42")  --> Error '42' happened

String functions

escape (s)
escape any Lua 'magic' characters in a string

Parameters:

  • s The input string
split (s, re, plain, n)
split a string into a list of strings separated by a delimiter.

Parameters:

  • s The input string
  • re optional A Lua string pattern; defaults to '%s+'
  • plain optional If truthy don't use Lua patterns
  • n optional maximum number of elements (if there are more, the last will remian un-split)

Returns:

    a list-like table

Raises:

error if s is not a string

See also:

splitv (s, re, plain, n)
split a string into a number of return values. Identical to split but returns multiple sub-strings instead of a single list of sub-strings.

Parameters:

  • s the string
  • re A Lua string pattern; defaults to '%s+'
  • plain don't use Lua patterns
  • n optional maximum number of splits

Returns:

    n values

See also:

Usage:

    first,next = splitv('user=jane=doe','=', false, 2)
    assert(first == "user")
    assert(next == "jane=doe")

Functional

memoize (func)
'memoize' a function (cache returned value for next call). This is useful if you have a function which is relatively expensive, but you don't know in advance what values will be required, so building a table upfront is wasteful/impossible.

Parameters:

  • func a function of at least one argument

Returns:

    a function with at least one argument, which is used as the key.
add_function_factory (mt, fun)
associate a function factory with a type. A function factory takes an object of the given type and returns a function for evaluating it

Parameters:

  • mt tab metatable
  • fun func a callable that returns a function
string_lambda (lf)
an anonymous function as a string. This string is either of the form '|args| expression' or is a function of one argument, '_'

Parameters:

  • lf function as a string

Returns:

    a function

Usage:

    string_lambda '|x|x+1' (2) == 3
    string_lambda '_+1' (2) == 3
bind1 (fn, p)
bind the first argument of the function to a value.

Parameters:

  • fn a function of at least two values (may be an operator string)
  • p a value

Returns:

    a function such that f(x) is fn(p,x)

Raises:

same as function_arg

See also:

Usage:

    local function f(msg, name)
      print(msg .. " " .. name)
    end
    
    local hello = utils.bind1(f, "Hello")
    
    print(hello("world"))     --> "Hello world"
    print(hello("sunshine"))  --> "Hello sunshine"
bind2 (fn, p)
bind the second argument of the function to a value.

Parameters:

  • fn a function of at least two values (may be an operator string)
  • p a value

Returns:

    a function such that f(x) is fn(x,p)

Raises:

same as function_arg

Usage:

    local function f(a, b, c)
      print(a .. " " .. b .. " " .. c)
    end
    
    local hello = utils.bind1(f, "world")
    
    print(hello("Hello", "!"))  --> "Hello world !"
    print(hello("Bye", "?"))    --> "Bye world ?"

Deprecation

set_deprecation_func (func)
Sets a deprecation warning function. An application can override this function to support proper output of deprecation warnings. The warnings can be generated from libraries or functions by calling utils.raise_deprecation. The default function will write to the 'warn' system (introduced in Lua 5.4, or the compatibility function from the compat module for earlier versions).

Note: only applications should set/change this function, libraries should not.

Parameters:

  • func a callback with signature: function(msg, trace) both arguments are strings, the latter being optional.

See also:

Usage:

    -- write to the Nginx logs with OpenResty
    utils.set_deprecation_func(function(msg, trace)
      ngx.log(ngx.WARN, msg, (trace and (" " .. trace) or nil))
    end)
    
    -- disable deprecation warnings
    utils.set_deprecation_func()
raise_deprecation (opts)
raises a deprecation warning. For options see the usage example below.

Note: the opts.deprecated_after field is the last version in which a feature or option was NOT YET deprecated! Because when writing the code it is quite often not known in what version the code will land. But the last released version is usually known.

Parameters:

  • opts options table

See also:

Usage:

    warn("@on")   -- enable Lua warnings, they are usually off by default
    
    function stringx.islower(str)
      raise_deprecation {
        source = "Penlight " .. utils._VERSION,                   -- optional
        message = "function 'islower' was renamed to 'is_lower'", -- required
        version_removed = "2.0.0",                                -- optional
        deprecated_after = "1.2.3",                               -- optional
        no_trace = true,                                          -- optional
      }
      return stringx.is_lower(str)
    end
    -- output: "[Penlight 1.9.2] function 'islower' was renamed to 'is_lower' (deprecated after 1.2.3, scheduled for removal in 2.0.0)"
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