Author:	haxscramper

This module implements pattern matching for objects, tuples, sequences, key-value pairs, case and derived objects. DSL can also be used to create object trees (AST).

Quick reference

Example	Explanation
(fld: @val)	Field fld into variable @val
Kind()	Object with .kind == Kind() [1]
of Derived()	Match object of derived type
(@val, _)	First element in tuple in @val
(@val, @val)	Tuple with two equal elements
{"key" : @val}	Table with "key", capture into @val [2]
[_, _]	Sequence with len == 2 [3]
[_, .._]	At least one element
[_, all @val]	All elements starting from index 1
[until @val == "2", .._]	Capture all elements until first "2" [4]
[until @val == 1, @val]	All including first match
[all @val == 12]	All elements are == 12, capture into @val
[some @val == 12]	At least one is == 12, capture all matching into @val

• [1] Kind fields can use shorted enum names - both nnkStrLit and StrLit will work (prefix nnk can be omitted)
• [2] Or any object with contains and [] defined (for necessary types)
• [3] Or any object with len proc or field
• [4] Note that sequence must match fully and it is necessary to have .._ at the end in order to accept sequences of arbitrary length.

Supported match elements

• seqs - matched using [Patt1(), Patt2(), ..]. Must have len(): int and iterator items(): T defined.
• tuples - matched using (Patt1(), Patt2(), ..).
• pairable - matched using {Key: Patt()}. Must have [Key]: T defined. Key is not a pattern - search for whole collection won't be performed.
• set - matched using {Val1, Val2, .._}. Must have contains defined. If variable is captured then Val1 must be comparable and collection should also implement items and incl.
• object - matched using (field: Val). Case objects are matched using Kind(field: Val). If you want to check agains multiple values for kind field (kind: in SomeSetOfKinds)

Element access

To determine whether particular object matches pattern access path is generated - sequence of fields and [] operators that you would normally write by hand, like fld.subfield["value"].len. Due to support for method call syntax there is no difference between field access and proc call, so things like (len: < 12) also work as expected.

(fld: "3") Match field fld against "3". Generated access

is expr.fld == "3".

["2"] Match first element of expression agains patt. Generate

acess expr[pos] == "2", where pos is an integer index for current position in sequence.

("2") For each field generate access using [1]

{"key": "val"} First check "key" in expr and then

expr["key"] == "val". No exception on missing keys, just fail match.

It is possible to have mixed assess for objects. Mixed object access via (gg: _, [], {}) creates the same code for checking. E.g ([_]) is the same as [_], ({"key": "val"}) and is identical to just {"key": "val"}. You can also call functions and check their values (like (len: _(it < 10)) or (len: in {0 .. 10})) to check for sequence length.

Checks

• Any operators with exception of is (subpattern) and of (derived object subpattern) is considered final comparison and just pasted as-is into generated pattern match code. E.g. fld: in {2,3,4} will generate expr.fld in {2,3,4}
• (fld: Patt()) - check if expr.fld matches pattern Patt()
• (fld: _.matchesPredicate()) - if call to matchesPredicate(expr.fld) evaluates to true.

Notation: <expr> refers to any possible combination of checks. For example

• fld: in {1,2,3} - <expr> is in {1,2,3}
• [_] - <expr> is _
• fld: Patt() - <expr> is Patt()

Examples

• (fld: 12) If rhs for key-value pair is integer, string or identifier then == comparison will be generated.
• (fld: == ident("33")) if rhs is a prefix of == then == will be generated. Any for of prefix operator will be converted to expr.fld <op> <rhs>.
• (fld: in {1, 3, 3}) or (fld: in Anything) creates fld.expr in Anything. Either in or notin can be used.

Variable binding

Match can be bound to new variable. All variable declarations happen via @varname syntax.

• To bind element to variable without any additional checks do: (fld: @varname)
• To bind element with some additional operator checks do:
  • (fld: @varname <operator> Value) first perform check using <operator> and then add Value to @varname - (fld: @hello is ("2" | "3"))
• Predicate checks: fld: @a.matchPredicate()
• Arbitrary expression: fld: @a(it mod 2 == 0). If expression has no type it is considered true.

Bind order

Bind order: if check evaluates to true variable is bound immediately, making it possible to use in other checks. [@head, any @tail != head] is a valid pattern. First match head and then any number of @tail elements. Can use any _(if it != head: tail.add it) and declare tail externally.

Variable is never rebound. After it is bound, then it will have the value of first binding.

Bind variable type

• Any variadics are mapped to sequence
• Only once in alternative is option
• Explicitly optional is option
• Optional with default value is regular value
• Variable can be used only once if in alternative

Pattern	Injected variables
[@a]	var a: typeof(expr[0])
{"key": @val}	var val: typeof(expr["key"])
[all @a]	var a: seq[typeof(expr[0])]
[opt @val]	var a: Option[typeof(expr[0])]
[opt @val or default]	var a: typeof(expr[0])
(fld: @val)	var val: typeof(expr.fld)

Matching different things

Sequence matching

Input sequence: [1,2,3,4,5,6,5,6]

Pattern	Result	Comment
[_]	Fail	Input sequence size mismatch
[.._]	Ok
[@a]	Fail	Input sequence size mismatch
[@a, .._]	Ok, a = 1
[any @a, .._]	Error
[any @a(it < 10)]	Ok, a = [1..6]	Capture all elements that match
[until @a == 6, .._]	Ok	All until first ocurrence of 6
[all @a == 6, .._]	Ok a = []	All leading 6
[any @a(it > 100)]	Fail	No elements > 100
[none @a(it in {6 .. 10})]	Fail	There is an element == 6
[0 .. 2 is < 10, .._]	Ok	First three elements < 10
[0 .. 2 is < 10]	Fail	Missing trailing .._

until

non-greedy. Match everything until <expr>

• until <expr>: match all until the first element that matches Expr

all

greedy. Match everything that matches <expr>

• all <expr>: all elements should match Expr
• all @val is <expr>: capture all elements in @val if <expr> is true for every one of them.

opt

Optional single element match - if sequence contains fewer elements than necessary element is considered missing. In that case either default fallback (if present) is used as value, or capture is set to None(T).

• opt @a: match optional element and bind it to a
• opt @a or "default": either match element to a or set a to "default"

any

greedy. Consume all sequence elements until the end and succeed only if at least one element has matched.

• any @val is "d": capture all element that match is "d"

none

greedy. Consume all sequence elements until the end and succed only if any element has matched. EE

[m .. n @capture]

Capture slice of elements from index m to n

Greedy patterns match until the end of a sequence and cannot be followed by anything else.

For sequence to match is must either be completely matched by all subpatterns or have trailing .._ in pattern.

Sequence	Pattern	Match result
[1,2,3]	[1,2] [1, .._] [1,2,_]	Fail Ok Ok

Use examples

• capture all elements in sequence: [all @elems]
• get all elements until (not including "d"): [until @a is "d"]
• All leading "d": [all @leading is "d"]
• Match first two elements and ignore the rest [_, _, .._]
• Match optional third element [_, _, opt @trail]
• Match third element and if not matched use default value [_, _, opt @trail or "default"]
• Capture all elements until first separator: [until @leading is "sep", @middle is "sep", all @trailing]
• Extract all conditions from IfStmt: IfStmt([all ElseIf([@cond, _]), .._])

In addition to working with nested subpatterns it is possible to use pattern matching as simple text scanner, similar to strscans. Main difference is that it allows working on arbitrary sequences, meaning it is possible, for example, to operate on tokens, or as in this example on strings (for the sake of simplicity).

func allIs(str: string, chars: set[char]): bool = str.allIt(it in chars)

"2019-10-11 school start".split({'-', ' '}).assertMatch([
  pref @dateParts(it.allIs({'0' .. '9'})),
  pref _(it.allIs({' '})),
  all @text
])

doAssert dateParts == @["2019", "10", "11"]
doAssert text == @["school", "start"]

Tuple matching

Input tuple: (1, 2, "fa")

Pattern	Result	Comment
(_, _, _)	Ok	Match all
(@a, @a, _)	Fail
(@a is (1 | 2), @a, _)	Fail	[1]
(1, 1 | 2, _)	Ok

• [1] Pattern backtracking is not performed, @a is first bound to 1, and in subsequent match attempts pattern fails.

Tuple element matches support any regular match expression like @capture, and not different from field matches. You can also use opt @capture or "default" in order to assign fallback value on tuple unpacking.

(@a, (@b, _), _) := ("hello", ("world", 11), 0.2)

Object matching

For matching object fields you can use (fld: value) -

type
  Obj = object
    fld1: int8

func len(o: Obj): int = 0

case Obj():
  of (fld1: < -10):
    discard

  of (len: > 10):
    # can use results of function evaluation as fields - same idea as
    # method call syntax in regular code.
    discard

  of (fld1: in {1 .. 10}):
    discard

  of (fld1: @capture):
    doAssert capture == 0

For objects with Option[T] fields it is possible to use field: opt @capture or "default" to either get capture value, or set it to fallback expression.

Variant object matching

Matching on .kind field is a very common operation and has special syntax sugar - ForStmt() is functionally equivalent to (kind: nnkForStmt), but much more concise.

nnk pefix can be omitted - in general if your enum field name folows nep1 naming conventions (each enum name starts with underscore prefix (common for all enum elements), followed PascalCase enum name.

Input AST

ForStmt
  Ident "i"
  Infix
    Ident ".."
    IntLit 1
    IntLit 10
  StmtList
    Command
      Ident "echo"
      IntLit 12

• ForStmt([== ident("i"), .._]) Only for loops with i as variable
• ForStmt([@a is Ident(), .._]) Capture for loop variable
• ForStmt([@a.isTuple(), .._]) for loops in which first subnode satisfies predicate isTuple(). Bind match to a
• ForStmt([_, _, (len: in {1 .. 10})]) between one to ten statements in the for loop body
• Using object name for pattern matching ObjectName() does not produce a hard error, but if .kind field does not need to be checked (fld: <pattern>) will be sufficient.
• To check .kind against multiple operators prefix in can be used - (kind: in {nnkForStmt, nnkWhileStmt})

Custom unpackers

It is possible to unpack regular object using tuple matcher syntax - in this case overload for [] operator must be provided that accepts static[FieldIndex] argument and returns a field.

type
  Point = object
    x: int
    y: int

proc `[]`(p: Point, idx: static[FieldIndex]): auto =
  when idx == 0:
    p.x
  elif idx == 1:
    p.y
  else:
    static:
      error("Cannot unpack `Point` into three-tuple")

let point = Point(x: 12, y: 13)

(@x, @y) := point

assertEq x, 12
assertEq y, 13

Note auto return type for [] proc - it is necessary if different types of fields might be returned on tuple unpacking, but not mandatory.

If different fields have varying types when must and static be used to allow for compile-time code selection.

Predicates and infix operators

Infix operators

By default object fields are either matched using recursive pattern, or compared for equality (when field: "some value" is used). It is also possible to explicitly specify operator, for example using =~ from std/pegs module:

It should be noted that implicitly injected matches variable is also visible in the case branch.

Custom predicates

Matching expressions using custom predicates is also possible. If it is not necessary to capture matched element placeholder _. should be used as a first argument:

proc lenEq(s: openarray[int], value: int): bool = s.len == value

case [1, 2]:
  of _.lenEq(3):
    # fails

  of _.lenEq(2):
    # matches

To capture value using predicate placeholder can be replaced with @capture pattern:

let arr = @[@[1, 2], @[2, 3], @[4]]
discard arr.matches([any @capture.lenEq(2)])
doAssert capture == @[@[1, 2], @[2, 3]]

Ref object matching

It is also possible to match derived ref objects with patterns using of operator. It allows for runtime selection of different derived types.

Note that of operator is necessary for distinguishing between multiple derived objects, or getting fields that are present only in derived types. In addition to it performs isNil() check in the object, so it might be used in cases when you are not dealing with derived types.

Due to isNil() check this pattern only makes sense when working with ref objects.

type
  Base1 = ref object of RootObj
    fld: int

  First1 = ref object of Base1
    first: float

  Second1 = ref object of Base1
    second: string

let elems: seq[Base1] = @[
  Base1(fld: 123),
  First1(fld: 456, first: 0.123),
  Second1(fld: 678, second: "test"),
  nil
]

for elem in elems:
  case elem:
    of of First1(fld: @capture1, first: @first):
      # Only capture `Frist1` elements
      doAssert capture1 == 456
      doAssert first == 0.123

    of of Second1(fld: @capture2, second: @second):
      # Capture `second` field in derived object
      doAssert capture2 == 678
      doAssert second == "test"

    of of Base1(fld: @default):
      # Match all *non-nil* base elements
      doAssert default == 123

    else:
      doAssert isNil(elem)

KV-pairs matching

Pattern matchig also support key-value pairs - any type that has [] and contains defined for the necessary types can be used. In this example we would use JsonNode type from the standard library.

Input json in all examples in this section (node variable)

{"menu": {
  "id": "file",
  "value": "File",
  "popup": {
    "menuitem": [
      {"value": "New", "onclick": "CreateNewDoc()",
       "ondrop": "OnDrop()"},
      {"value": "Open", "onclick": "OpenDoc()"},
      {"value": "Close", "onclick": "CloseDoc()"}
    ]
  }
}}

Get each "value" from an array. Resulting match would be stored values variable - @[%"New", %"Open", %"Close"]

It is also possible to mix key-value pairs, field and kind object matching. In this example case first branch would trigger if node contains "value" that is a jstring, with value "File". Second would trigger if string value is anything else (but it must still be a jstring).

Collect "ondrop" from all elements in array, providing fallback values - drops would contain @[%"OnDrop()", %"<defaulted>", %"<defaulted>"]

Collect only explicitly specified values - capture @[%"OnDrop()"]

Option matching

Some(@x) and None() is a special case that will be rewritten into (isSome: true, get: @x) and (isNone: true) respectively. This is made to allow better integration with optional types. [9]_ .

Note: implementation does not explicitly require to use std/options.Option type, but instead works with anything that provides following functions:

• isSome(): bool (for Some() pattern check),
• isNone(): bool (for None() pattern), and
• get(): T (for getting value if type is some).

Some() pattern can be used with ?= to unpack optionals in conditions:

for it in @[some(12), none(int)]:
  if Some(@unpacked) ?= it:
    doAssert unpacked == 12

Difference between Some()/None() and opt

Some() and None() checks are used only when working with Option type (and any that provides the same API). When such pattern is encountered it is immediately transformed into isSome/isNone calls.

opt on the other hand is used for dealing with potentially missing values and providing default fallback values. In sequences, tables or optional fields. When used opt might add one layer of optionality if default value is not provided, or remove one layer if value is provided.

Tree matching

For deeply nested AST structures it might be really inconvenient to write one-line expression with lots of ProcDef[@name is Ident() | Postfix[_, @name is Ident()]] and so on. But it is possible to use block syntax for patterns if necessary -

ProcDef:
  @name is Ident() | Postfix[_, @name is Ident()]
  # Other pattern parts

In case of ProcDef: pattern braces can be omitted because it is clear that we are trying to match a case object here.

Tree matching syntax has a nice property of being extremely close (copy-pastable) from treeRepr for NimNode. For a following proc declaration:

proc testProc1(arg1: int) {.unpackProc.} =
  discard

We have an ast

ProcDef
  Ident "testProc1"
  Empty
  Empty
  FormalParams
    Empty
    IdentDefs
      Ident "arg1"
      Ident "int"
      Empty
  Empty
  Empty
  StmtList
    DiscardStmt
      Empty

That can be matched using following pattern:

Tree construction

makeTree provides 'reversed' implementation of pattern matching, which allows to construct tree from pattern, using variables. Example of use

type
  HtmlNodeKind = enum
    htmlBase = "base"
    htmlHead = "head"
    htmlLink = "link"

  HtmlNode = object
    kind*: HtmlNodeKind
    text*: string
    subn*: seq[HtmlNode]

func add(n: var HtmlNode, s: HtmlNode) = n.subn.add s

discard makeTree(HtmlNode):
  base:
    link(text: "hello")

In order to construct tree, kind= and add have to be defined. Internally DSL just creats resulting object, sets kind= and then repeatedly add elements to it. In order to properties for objects either the field has to be exported, or fld= has to be defined (where fld is the name of property you want to set).