diff options
Diffstat (limited to 'lib/std/wrapnils.nim')
| -rw-r--r-- | lib/std/wrapnils.nim | 187 |
1 files changed, 136 insertions, 51 deletions
diff --git a/lib/std/wrapnils.nim b/lib/std/wrapnils.nim index 3ff48fbfe..0b75c270e 100644 --- a/lib/std/wrapnils.nim +++ b/lib/std/wrapnils.nim @@ -1,9 +1,20 @@ -## This module allows chains of field-access and indexing where the LHS can be nil. -## This simplifies code by reducing need for if-else branches around intermediate values -## that may be nil. +## This module allows evaluating expressions safely against the following conditions: +## * nil dereferences +## * field accesses with incorrect discriminant in case objects +## +## `default(T)` is returned in those cases when evaluating an expression of type `T`. +## This simplifies code by reducing need for if-else branches. ## ## Note: experimental module, unstable API. +#[ +TODO: +consider handling indexing operations, eg: +doAssert ?.default(seq[int])[3] == default(int) +]# + +import std/macros + runnableExamples: type Foo = ref object x1: string @@ -24,8 +35,125 @@ runnableExamples: assert (?.f2.x2.x2).x3 == nil # this terminates ?. early +runnableExamples: + # ?. also allows case object + type B = object + b0: int + case cond: bool + of false: discard + of true: + b1: float + + var b = B(cond: false, b0: 3) + doAssertRaises(FieldDefect): discard b.b1 # wrong discriminant + doAssert ?.b.b1 == 0.0 # safe + b = B(cond: true, b1: 4.5) + doAssert ?.b.b1 == 4.5 + + # lvalue semantics are preserved: + if (let p = ?.b.b1.addr; p != nil): p[] = 4.7 + doAssert b.b1 == 4.7 + +proc finalize(n: NimNode, lhs: NimNode, level: int): NimNode = + if level == 0: + result = quote: `lhs` = `n` + else: + result = quote: (let `lhs` = `n`) + +proc process(n: NimNode, lhs: NimNode, label: NimNode, level: int): NimNode = + var n = n.copyNimTree + var it = n + let addr2 = bindSym"addr" + var old: tuple[n: NimNode, index: int] + while true: + if it.len == 0: + result = finalize(n, lhs, level) + break + elif it.kind == nnkCheckedFieldExpr: + let dot = it[0] + let obj = dot[0] + let objRef = quote do: `addr2`(`obj`) + # avoids a copy and preserves lvalue semantics, see tests + let check = it[1] + let okSet = check[1] + let kind1 = check[2] + let tmp = genSym(nskLet, "tmpCase") + let body = process(objRef, tmp, label, level + 1) + let tmp3 = nnkDerefExpr.newTree(tmp) + it[0][0] = tmp3 + let dot2 = nnkDotExpr.newTree(@[tmp, dot[1]]) + if old.n != nil: old.n[old.index] = dot2 + else: n = dot2 + let assgn = finalize(n, lhs, level) + result = quote do: + `body` + if `tmp3`.`kind1` notin `okSet`: break `label` + `assgn` + break + elif it.kind in {nnkHiddenDeref, nnkDerefExpr}: + let tmp = genSym(nskLet, "tmp") + let body = process(it[0], tmp, label, level + 1) + it[0] = tmp + let assgn = finalize(n, lhs, level) + result = quote do: + `body` + if `tmp` == nil: break `label` + `assgn` + break + elif it.kind == nnkCall: # consider extending to `nnkCallKinds` + # `copyNimTree` needed to avoid `typ = nil` issues + old = (it, 1) + it = it[1].copyNimTree + else: + old = (it, 0) + it = it[0] + +macro `?.`*(a: typed): auto = + ## Transforms `a` into an expression that can be safely evaluated even in + ## presence of intermediate nil pointers/references, in which case a default + ## value is produced. + let lhs = genSym(nskVar, "lhs") + let label = genSym(nskLabel, "label") + let body = process(a, lhs, label, 0) + result = quote do: + var `lhs`: type(`a`) + block `label`: + `body` + `lhs` + +# the code below is not needed for `?.` from std/options import Option, isSome, get, option, unsafeGet, UnpackDefect -export options.get, options.isSome, options.isNone + +macro `??.`*(a: typed): Option = + ## Same as `?.` but returns an `Option`. + runnableExamples: + import std/options + type Foo = ref object + x1: ref int + x2: int + # `?.` can't distinguish between a valid vs invalid default value, but `??.` can: + var f1 = Foo(x1: int.new, x2: 2) + doAssert (??.f1.x1[]).get == 0 # not enough to tell when the chain was valid. + doAssert (??.f1.x1[]).isSome # a nil didn't occur in the chain + doAssert (??.f1.x2).get == 2 + + var f2: Foo + doAssert not (??.f2.x1[]).isSome # f2 was nil + + doAssertRaises(UnpackDefect): discard (??.f2.x1[]).get + doAssert ?.f2.x1[] == 0 # in contrast, this returns default(int) + + let lhs = genSym(nskVar, "lhs") + let lhs2 = genSym(nskVar, "lhs") + let label = genSym(nskLabel, "label") + let body = process(a, lhs2, label, 0) + result = quote do: + var `lhs`: Option[type(`a`)] + block `label`: + var `lhs2`: type(`a`) + `body` + `lhs` = option(`lhs2`) + `lhs` template fakeDot*(a: Option, b): untyped = ## See top-level example. @@ -59,50 +187,7 @@ func `[]`*[U](a: Option[U]): auto {.inline.} = if a2 != nil: result = option(a2[]) -import std/macros - -func replace(n: NimNode): NimNode = - if n.kind == nnkDotExpr: - result = newCall(bindSym"fakeDot", replace(n[0]), n[1]) - elif n.kind == nnkPar: - doAssert n.len == 1 - result = newCall(bindSym"option", n[0]) - elif n.kind in {nnkCall, nnkObjConstr}: - result = newCall(bindSym"option", n) - elif n.len == 0: - result = newCall(bindSym"option", n) - else: - n[0] = replace(n[0]) - result = n - -proc safeGet[T](a: Option[T]): T {.inline.} = - get(a, default(T)) - -macro `?.`*(a: untyped): auto = - ## Transforms `a` into an expression that can be safely evaluated even in - ## presence of intermediate nil pointers/references, in which case a default - ## value is produced. - result = replace(a) - result = quote do: - # `result`.val # TODO: expose a way to do this directly in std/options, e.g.: `getAsIs` - safeGet(`result`) - -macro `??.`*(a: untyped): Option = - ## Same as `?.` but returns an `Option`. - runnableExamples: - type Foo = ref object - x1: ref int - x2: int - # `?.` can't distinguish between a valid vs invalid default value, but `??.` can: - var f1 = Foo(x1: int.new, x2: 2) - doAssert (??.f1.x1[]).get == 0 # not enough to tell when the chain was valid. - doAssert (??.f1.x1[]).isSome # a nil didn't occur in the chain - doAssert (??.f1.x2).get == 2 - - var f2: Foo - doAssert not (??.f2.x1[]).isSome # f2 was nil - from std/options import UnpackDefect - doAssertRaises(UnpackDefect): discard (??.f2.x1[]).get - doAssert ?.f2.x1[] == 0 # in contrast, this returns default(int) - - result = replace(a) +when false: + # xxx: expose a way to do this directly in std/options, e.g.: `getAsIs` + proc safeGet[T](a: Option[T]): T {.inline.} = + get(a, default(T)) |