alike is similar to
all.equal from base R except it only compares object structure. As with
all.equal, the first argument (
target) must be matched by the second (
 "`length(1:4)` should be 5 (is 4)"
 "`letters` should be type \"integer-like\" (is \"character\")"
alike only compares structural elements that are defined in
target (a.k.a. the template). This allows “wildcard” templates. For example, we consider length zero vectors to have undefined length so those match vectors of any length:
 "`letters` should be type \"integer-like\" (is \"character\")"
Similarly, if a template does not specify an attribute, objects with any value for that attribute will match:
 "`list()` should be class \"data.frame\" (is \"list\")"
As an extension to the wildcard concept, we interpret partially specified core R attributes. Here we allow any three column integer matrix to match:
 "`matrix(sample(1:12), nrow = 3)` should have 3 columns (has 4)"
or a data frame of arbitrary number of rows, but same column structure as
 "`names(CO2)` should be \"Sepal.Length\" (is \"Plant\")"
“alikeness” is complex to describe, but should be intuitive to grasp. We recommend you look
example(alike) to get a sense of “alikeness”. If you want to understand the specifics, read on.
alike’s template based comparison is declarative. You declare what structure an object is expected to implement, and
vetr infers all the computations required to verify that is so. This makes is particularly well suited for enforcing structural requirements for S3 objects. The S4 system does this and more, but S3 objects are still used extensively in R code, and sometimes S4 classes are not appropriate.
There are several advantages to template based comparisons:
The template concept was inspired by
alike compares objects on type, length, and attributes. Recursive structures are compared element by element. Language objects and functions are compared specially because the concept of a value within those is more complex (e.g., is the
x + y just a value?).
We will defer discussion of attribute comparison to the attributes section.
Objects must be the same length to be
alike, unless the template (
target) is zero length, in which case the object may be any length. Environments are an exception: we only require that all the elements present in
target be present in
current. Also, note that calls to
( are ignored in language objects, which may affect length computation.
Type comparison is done on type (i.e. the
typeof) with some adjustments to better align comparisons to “percieved” types as opposed to internal storage types.
We allow integer vectors to be considered numeric, and short integer-like numerics to be treated as integers:
 "`1.1` should be type \"integer-like\" (is \"double\")"
This feature is designed to simplify checks for integer-like numbers. The following two expressions are roughly equivalent:
Note that we only check numerics of length <= 100 for integerness to avoid full scans on large vectors. We expect that the primary source of these integer-like numerics is hand input vectors (e.g.
c(1, 2, 3)), so hopefully this compromise is not too limiting. You can modify the threshold length for this treatment via the
fuzzy.int.max.len parameter to the
settings objects (see
Closures, builtins, and specials are all treated as a single type, even though internally they are stored as different types.
alike will recurse through lists (and by extension data frames), pairlists, expressions, and environments and will check pairwise alikeness between the corresponding elements of the
currentmay have additional items
currentmust be too (this is because the global environment is often littered with many objects, and explicitly comparing it to another environment could be computationally expensive)
NULL elements within templates in recursive objects are considered undefined and as such act like wildcards:
 "`length(list(1:10, letters, iris))` should be 2 (is 3)"
Note that top level
NULLs do not act as wildcards:
 "`1:10` should be `NULL` (is \"integer\")"
NULL inconsistently depending on whether it is nested or not is a compromise designed to make
alike a better fit for argument validation because arguments that are
NULL by default are fairly common.
alike will check for self-referential loops in nested environments and prevent infinite recursion. If you somehow introduce a self-referential structure in a template without using environments then
alike will get stuck in an infinite recursion loop.
We are currently considering adding new comparison modes for lists that would allow for checks more similar to environments (see #29).
Alikeness for these types of objects is a little harder to define. We have settled on somewhat arbitrary semantics, though hopefully they are intuitive. These may change in the future as we gain experience using
alike with these types of objects. This is particularly true of functions.
Language objects are also compared recursively, but alikeness has a slightly different meaning for them:
 "`quote(sum(x, x))[]` should not be `x`"
 "`quote(sum(x, y))[]` should be a call to `mean` (is a call to `sum`)"
Since variables can contain anything we do not require them to match directly across calls. In the examples above the second call fails because the template defines different variables for each argument, but the
current object uses the same variable twice. The third call fails because the functions are different and as such the calls are fundamentally different.
If a function is defined in the calling frame,
match.call it prior to testing alikeness:
 "`quote(fun(y, x, x))[]` should be `y` (is `x`)"
Constants match any constants, but keep in mind that expressions like
c(1, 2, 3) are calls to
c respectively, not constants in the context of language objects.
NULL is a wild card in calls as well:
 "`quote(log(10, 10))` should have 1 arguments (has 2)"
( are ignored when comparing calls since parentheses are redundant in call trees because the tree structure encodes operation precedence independent of operator precedence.
We concede that the rules for “alikeness” of language objects are arbitrary, but hope the outcomes of those rules is generally intuitive. Unfortunately value and structure are somewhat intertwined for language objects so we must impose our own view of what is value and what is structure.
Formulas are treated like calls, except that constants must match:
 "`(a ~ b^3)[][]` should have identical constant values"
alike if the signature of the
current function can reasonably be interpreted as a valid method for the
 "`print` should have argument `digits` after argument `x`"
A method of a generic must have all arguments present in the generic, with the same default values if those are defined. If the generic contains
... then the method may have additional arguments, but must also contain
Potential changes / improvements for function comparison are being considered in #35.
S4 and RC objects are considered alike if
current inherits from
class(target). Since these objects embed structural information in their definitions
alike relies on class alone to establish alikeness.
Objects of the following types are actually references to specific memory locations:
These are typically attached as attributes to other objects that contain the information required to establish alikeness (e.g.
data.table, byte-compiled functions), so we only check their type.
Much of the structure of an object is determined by attributes.
alike recursively compares object attributes and requires them to be
alike, unless the attribute is a special attribute or an environment. Environments within attributes in the template must be matched by an environment, but nothing is checked about the environments to avoid expensive computations on objects that commonly include environments in their attributes (e.g. formulas); note this is different than the treatment of environments as actual objects.
Only attributes present in the template object are checked:
 "`structure(logical(1L), a = integer(3L))` should have attribute \"b\""
Attributes present in
current but missing in
target may be anything at all.
The special attributes are
levels. These attributes are discussed in sections 2.2 and 2.3 of the R Language Definition, and have well defined and consistently applied semantics in R. Since the semantics of these attributes are well known, we are able to define “alikeness” for them in a more granular way than we can for arbitrary attributes.
We also consider
srcref to be a special attribute. This attribute is not checked.
If present in
target, then must be matched exactly by the corresponding attribute in
current, except that:
character(0L)) will match any character
"") in a
row.namescharacter vector will allow any value to match at the corresponding position of the
 "`1:3` should have attribute \"names\""
 "`names(c(a = 1, b = 2, c = 3))` should be \"Z\" (is \"c\")"
dim attributes must be identical between
current, except that if a value of the
dim vector is zero in
target then the corresponding value in
current can be any value. This is how comparisons like the following succeed:
 "`matrix(sample(1:12), nrow = 3)` should have 3 columns (has 4)"
int[0 , 1:3]
Must also be identical, except that if the
target value of the
dimnames list for a particular dimension is
NULL, then the corresponding
dimnames value in
current may be anything. As with
names, zero character
dimname element elements match any name.
 "`dimnames(mx.cur2)` should have attribute \"names\""
dimnames can have a
names attribute. This
names attributed is treated as described in row.names and names.
 "row.id" ""
S3 objects are considered alike if the
current class inherits from the
target class. Note that “inheritance” here is used in a stricter context than in the typical S3 application:
targetmust be present in
currentmust be the same as the last class in
 "`class(cur2)` should be \"a\" (is \"b\")"
tsp attribute of
ts objects behaves similarly to the
dim attribute. Any component (i.e. start, end, frequency) that is set to zero will act as a wild card. Other components must be identical. It is illegal to set
tsp components to zero throught the standard R interface, but you may use
abstract as a work-around.
Levels are compared like row.names and names.
This attribute is completely ignored.
If an object contains one of the special attributes, but the attribute value is inconsistent with the standard definition of the attribute,
alike will silently treat that attribute as any other normal attribute.
You can use the
settings parameter to
alike to modify comparison behavior. See
?vetr_settings for details.
You can always create your own templates by manually building R structures:
Alternatively, you can start with a known structure, and abstract away the instance-specific details. For example, suppose we are sending sample collectors out on the field to record information about iris flowers:
abstract is an S3 generic defined by
alike along with methods for common objects.
abstract primarily sets the
length of atomic vectors to zero:
[] named numeric(0) [] character(0)
and also abstracts the
tsp attributes if present. Other attributes are left untouched unless a specific
abstract method exists for a particular object that also modifies attributes. One example of such a method is
abstract.lm, and it does some minor tweaking to the base abstractions to allow us to match models produced by
 "`lm(Sepal.Length ~ Sepal.Width, iris)$terms[]` should be a call to `+` (is \"symbol\")"
The error message is telling us that at index
lm(Sepal.Length ~ Sepal.Width, iris)$terms)
alike was expecting a call to
+ instead of a symbol (i.e
Sepal.Width + <somevar> instead of
Sepal.Width). The message could certainly be more eloquent, but with a little context it should provide enough information to figure out the problem.
We have gone to great lengths to make
alike fast so that it can be included in other functions without concerns for what overhead:
Mean eval time from 10000 iterations, in microseconds: identical(rivers, rivers) ~ 0.6 alike(rivers, rivers) ~ 3.1 type_and_len(rivers, rivers) ~ 2.0
alike is slower than
identical and the comparable bare bones R function, it is competitive with a bare bones R function that checks types and length. As objects grow more complex,
identical will obviously pull ahead, though
alike should be sufficiently fast for most applications:
Mean eval time from 10000 iterations, in microseconds: identical(mtcars, mtcars) ~ 0.5 alike(mtcars, mtcars) ~ 25.7
In the above example, we are comparing the data frames, their attributes, and the 11 columns individually.
Keep in mind that the complexity of the
alike comparison is driven by the complexity of the template, not the object we are checking, so we can always manage the expense of the
Comparisons that succeed will be substantially faster than comparisons that fail as the construction of error messages is non-trivial and we have prioritized optimization in the success case.
Language object comparison is relatively slow. We intend to optimize this some day.
Templates with large numbers of attributes (e.g. > 25) may scale non-linearly. We intend to optimize this some day, though in our experience objects with that many attributes are rare (note having multiple objects each with a handful attributes nested in recursive structures is not a problem).
Large objects will be slower to evaluate. Let us revisit the
lm example, though this time we compare our template to itself to ensure that the comparisons succeed for
Mean eval time from 1000 iterations, in microseconds: alike(mdl.tpl, mdl.tpl) ~ 210.5 all.equal(mdl.tpl, mdl.tpl) ~ 2866.0 identical(mdl.tpl, mdl.tpl) ~ 0.5
Even with template as large as
lm results (check
str(mdl.tpl)) we can evaluate
alike thousands of times before the overhead becomes noticeable.
Some fairly innocuous R expressions carry substantial overhead. Consider:
Mean eval time from 1000 iterations, in microseconds: alike(df.tpl, df.cur) ~ 12 alike(data.frame(integer(), numeric()).. ~ 406
data.frame is a particularly slow constructor, but in general you are best served by defining your templates (including calls to
abstract) outside of your function so they are created on package load rather than every time your function is called.
alikeas an S3 generic
alike is not currently an S3 generic, but will likely one in the future provided we can create an implementation with and acceptable performance profile.