+
+
+
+
+
Example:
+
import src/fusion/matching
+{.experimental: "caseStmtMacros".}
+
+case [(1, 3), (3, 4)]:
+ of [(1, @a), _]:
+ echo a
+
+ else:
+ echo "Match failed"
+"you can probably make a macro for that" -- Rika, 22-09-2020 10:41:51
+
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).
+
+
| 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.
+
+
+
- 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)
+
+
+
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.
+
+
- 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()
+
+
+
- (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.
+
+
+
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: 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.
+
+
- 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) |
+
+
+
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 |
+
+
- 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"]
+
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)
+
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):
+
+
+ discard
+
+ of (fld1: in {1 .. 10}):
+ discard
+
+ of (fld1: @capture):
+ doAssert capture == 0For 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.
+
+
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})
+
+
+
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, 13Note 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.
+
+
+
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:
+
case (parent: (field: "string")):
+ of (parent.field: =~ peg"str{\w+}"):
+ doAssert matches[0] == "ing"It should be noted that implicitly injected matches variable is also visible in the case branch.
+
+
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):
+
+
+ of _.lenEq(2):
+
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]]
+
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):
+
+ doAssert capture1 == 456
+ doAssert first == 0.123
+
+ of of Second1(fld: @capture2, second: @second):
+
+ doAssert capture2 == 678
+ doAssert second == "test"
+
+ of of Base1(fld: @default):
+
+ doAssert default == 123
+
+ else:
+ doAssert isNil(elem)
+
Input json string
+
{"menu": {
+ "id": "file",
+ "value": "File",
+ "popup": {
+ "menuitem": [
+ {"value": "New", "onclick": "CreateNewDoc()"},
+ {"value": "Open", "onclick": "OpenDoc()"},
+ {"value": "Close", "onclick": "CloseDoc()"}
+ ]
+ }
+}}- Get input ["menu"]["file"] from node and
+
+
case inj:
+ of {"menu" : {"file": @file is JString()}}:
+
+ else:
+ raiseAssert("Expected [menu][file] as string, but found " & $inj)
+
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).
+
+
+
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()]
+
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.} =
+ discardWe 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:
+
procDecl.assertMatch:
+ ProcDef:
+ Ident(strVal: @name) | Postfix[_, Ident(strVal: @name)]
+ _
+ _
+ FormalParams:
+ @returnType
+ all IdentDefs[@trailArgsName, _, _]
+
+ @pragmas
+ _
+ @implementation
+
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).
+
+
+
+
+
+
+
ItemMatchKind = enum
+ imkInfixEq,
+ imkSubPattern,
+ imkPredicate
+
+
-
+
+ Type of item pattern match
+
+
+
+
Match = ref object
+ bindVar*: Option[NimNode]
+ declNode*: NimNode
+ isOptional*: bool
+ fallback*: Option[NimNode]
+ case kind*: MatchKind
+ of kItem:
+ case
+ of imkInfixEq:
+ infix*: string
+ rhsNode*: NimNode
+ isPlaceholder*: bool
+
+ of imkSubPattern:
+ rhsPattern*: Match
+ of imkPredicate:
+ isCall*: bool
+
+
+ predBody*: NimNode
+ of kAlt:
+ altElements*: seq[Match]
+ of kSeq:
+ seqElements*: seq[SeqStructure]
+ of kTuple:
+ tupleElements*: seq[Match]
+ of kPairs:
+ pairElements*: seq[KVPair]
+ nocheck*: bool
+ of kSet:
+ setElements*: seq[Match]
+ of kObject:
+ kindCall*: Option[NimNode]
+
+ isRefKind*: bool
+ fieldElements*: seq[tuple[name: string, pattern: Match]]
+ kvMatches*: Option[Match]
+
+ seqMatches*: Option[Match]
+
+
-
+
+ Object describing single match for element
+
+
+
+
MatchError = ref object of CatchableError
+
-
+
+ Exception indicating match failure
+
+
+
+
MatchKind = enum
+ kItem,
+ kSeq,
+ kTuple,
+ kPairs,
+ kObject,
+ kSet,
+ kAlt
+
-
+
+ Different kinds of matching patterns
+
+
+
+
SeqKeyword = enum
+ lkAny = "any",
+ lkAll = "all",
+ lkNone = "none",
+ lkOpt = "opt",
+ lkUntil = "until",
+ lkPref = "pref",
+ lkPos,
+ lkSlice,
+ lkTrail
+
-
+
+ Possible special words for seq pattern matching
+
+
+
+
SeqStructure = object
+
+ bindVar*: Option[NimNode]
+ pattern*: Match
+ case kind*: SeqKeyword
+ of lkSlice:
+ slice*: NimNode
+ else:
+ nil
+
-
+
+
+
+
+
+
VarKind = enum
+ vkRegular,
+ vkSequence, vkOption, vkSet, vkAlt
+
-
+
+ Kind of matched variables
+
+
+
+
VarSpec = object
+ decl*: NimNode
+ case varKind*: VarKind
+ of vkAlt:
+ prefixMap*: Table[Path, AltSpec]
+ else:
+ nil
+ typePath*: Path
+
+ foundCount*: int
+
-
+
+
+
+
+
+
+
+
+
+
+
nnkFloatKinds = {nnkFloatLit..nnkFloat128Lit}
+
-
+
+ Set of all nim node kinds for float literal nodes
+
+
+
+
nnkIdentKinds = {nnkIdent, nnkSym, nnkOpenSymChoice}
+
-
+
+ Set of all nim node kinds for identifier-like nodes
+
+
+
+
nnkIntKinds = {nnkCharLit..nnkUInt64Lit}
+
-
+
+ Set of all nim node kinds for integer literal nodes
+
+
+
+
nnkStrKinds = {nnkStrLit..nnkTripleStrLit}
+
-
+
+ Set of all nim node kinds for string nodes
+
+
+
+
nnkTokenKinds = {nnkEmpty..nnkSym, nnkCharLit..nnkTripleStrLit, nnkOpenSymChoice}
+
-
+
+ Set of all token-like nodes (primitive type literals or identifiers)
+
+
+
+
+
+
+
+
+
+
func `$`(kvp: KVPair): string {....raises: [Exception], tags: [RootEffect],
+ forbids: [].}
+
-
+
+
+
+
+
+
+
func `$`(match: Match): string {....raises: [Exception], tags: [RootEffect],
+ forbids: [].}
+
-
+
+
+
+
+
+
+
func `$`(path: Path): string {....raises: [], tags: [], forbids: [].}
+
-
+
+
+
+
+
+
+
func `$`(ss: SeqStructure): string {....raises: [Exception], tags: [RootEffect],
+ forbids: [].}
+
-
+
+
+
+
+
+
+
+
+
proc getKindNames(head: NimNode): (string, seq[string]) {....raises: [KeyError],
+ tags: [], forbids: [].}
+
-
+
+
+
+
+
+
+
+
+
func kind=(node: var NimNode; kind: NimNodeKind) {....raises: [], tags: [],
+ forbids: [].}
+
-
+
+
+
+
+
+
+
+
+
func makeMatchExpr(m: Match; mainExpr: NimNode; doRaise: bool;
+ originalMainExpr: NimNode): tuple[expr: NimNode,
+ vtable: VarTable, mixident: seq[string]] {....raises: [KeyError, Exception],
+ tags: [RootEffect], forbids: [].}
+
-
+
+ Create NimNode for checking whether or not item referred to by mainExpr matches pattern described by Match
+
+
+
+
+
+
+
func parseMatchExpr(n: NimNode): Match {....raises: [Exception],
+ tags: [RootEffect], forbids: [].}
+
-
+
+
+
+
+
+
+
+
+
func str(node: NimNode): string {....raises: [], tags: [], forbids: [].}
+
-
+
+
+
+
+
+
+
+
+
func str=(node: var NimNode; val: string) {....raises: [], tags: [], forbids: [].}
+
-
+
+
+
+
+
+
+
+
+
func toAccs(path: Path; name: NimNode; pathForType: bool): NimNode {....raises: [],
+ tags: [], forbids: [].}
+
-
+
+ Convert path in object to expression for getting element at path.
+
+
+
+
+
+
+
+
+
+
+
+
macro assertMatch(input, pattern: untyped): untyped
+
-
+
+ Try to match input using pattern and raise MatchError on failure. For DSL syntax details see start of the document.
+
+
+
+
+
+
+
macro `case`(n: untyped): untyped
+
-
+
+
+
+
+
+
+
+
+
macro expand(body: typed): untyped
+
-
+
+
+
+
+
+
+
+
+
macro hasKindImpl(head: typed; kind: untyped): untyped
+
-
+
+
+
+
+
+
+
+
+
macro matches(input, pattern: untyped): untyped
+
-
+
+ Try to match input using pattern and return false on failure. For DSL syntax details see start of the document.
+
+
+
+
+
+
+
+
+
+
+
+
template `:=`(lhs, rhs: untyped): untyped
+
-
+
+ Shorthand for assertMatch
+
+
+
+
+
+
+
template `?=`(lhs, rhs: untyped): untyped
+
-
+
+ Shorthand for matches
+
+
+
+
+
+
+
template `[]`(t: tuple; idx: static[FieldIndex]): untyped
+
-
+
+
+
+
+
+
+
+
+
template hasKind(head, kindExpr: untyped): untyped
+
-
+
+ Determine if head has kind value. Either function/procedure kind or field with the same name is expected to be declared. Type of kind must be an enum. Kind expression is a pattern describing expected values. Possible examples of pattern (assuming value of type NimNode is used as head)
- nnkIntLit - match integer literal
+- IntLit - alternative (preferred) syntax for matching enum values nnk prefix can be omitted.
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template makeTree(T: typed; pattern: untyped): untyped
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+ Construct tree from pattern matching expression. For example of use see documentation at the start of the module
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