Add optionals module (Maybe[T])

lib/pure/optionals.nim

@@ -0,0 +1,262 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2015 Nim Contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## :Author: Oleh Prypin
+##
+## Abstract
+## ========
+##
+## This module implements types which encapsulate an optional value.
+##
+## A value of type ``?T`` (``Maybe[T]``) either contains a value `x`
+## (represented as ``just(x)``) or is empty (``nothing(T)``).
+##
+## This can be useful when you have a value that can be present or not.
+## The absence of a value is often represented by ``nil``, but it is not always
+## available, nor is it always a good solution.
+##
+##
+## Tutorial
+## ========
+##
+## Let's start with an example: a procedure that finds the index of a character
+## in a string.
+##
+## .. code-block:: nim
+##
+##   import optionals
+##
+##   proc find(haystack: string, needle: char): ?int =
+##     for i, c in haystack:
+##       if c == needle:
+##         return just i
+##     return nothing(int)  # This line is actually optional,
+##                          # because the default is empty
+##
+## The ``?`` operator (template) is a shortcut for ``Maybe[T]``.
+##
+## .. code-block:: nim
+##
+##   try:
+##     assert("abc".find('c')[] == 2)  # Immediately extract the value
+##   except FieldError:  # If there is no value
+##     assert false  # This will not be reached, because the value is present
+##
+## The ``[]`` operator demonstrated above returns the underlying value, or
+## raises ``FieldError`` if there is no value. There is another option for
+## obtaining the value: ``val``, but you must only use it when you are
+## absolutely sure the value is present (e.g. after checking ``has``). If you do
+## not care about the tiny overhead that ``[]`` causes, you should simply never
+## use ``val``.
+##
+## How to deal with an absence of a value:
+##
+## .. code-block:: nim
+##
+##   let result = "team".find('i')
+##
+##   # Nothing was found, so the result is `nothing`.
+##   assert(result == nothing(int))
+##   # It has no value:
+##   assert(result.has == false)
+##   # A different way to write it:
+##   assert(not result)
+##
+##   try:
+##     echo result[]
+##     assert(false)  # This will not be reached
+##   except FieldError:  # Because an exception is raised
+##     discard
+##
+## Now let's try out the extraction template. It returns whether a value
+## is present and injects the value into a variable. It is meant to be used in
+## a conditional.
+##
+## .. code-block:: nim
+##
+##   if pos ?= "nim".find('i'):
+##     assert(pos is int)  # This is a normal integer, no tricks.
+##     echo "Match found at position ", pos
+##   else:
+##     assert(false)  # This will not be reached
+##
+## Or maybe you want to get the behavior of the standard library's ``find``,
+## which returns `-1` if nothing was found.
+##
+## .. code-block:: nim
+##
+##   assert(("team".find('i') or -1) == -1)
+##   assert(("nim".find('i') or -1) == 1)
+
+import typetraits
+
+
+type
+  Maybe*[T] = object
+    ## An optional type that stores its value and state separately in a boolean.
+    val: T
+    has: bool
+
+
+template `?`*(T: typedesc): typedesc =
+  ## ``?T`` is equivalent to ``Maybe[T]``.
+  Maybe[T]
+
+
+proc just*[T](val: T): Maybe[T] =
+  ## Returns a ``Maybe`` that has this value.
+  result.has = true
+  result.val = val
+
+proc nothing*(T: typedesc): Maybe[T] =
+  ## Returns a ``Maybe`` for this type that has no value.
+  result.has = false
+
+
+proc has*(maybe: Maybe): bool =
+  ## Returns ``true`` if `maybe` isn't `nothing`.
+  maybe.has
+
+converter toBool*(maybe: Maybe): bool =
+  ## Same as ``has``. Allows to use a ``Maybe`` in boolean context.
+  maybe.has
+
+
+proc unsafeVal*[T](maybe: Maybe[T]): T =
+  ## Returns the value of a `just`. Behavior is undefined for `nothing`.
+  assert maybe.has, "nothing has no val"
+  maybe.val
+
+proc `[]`*[T](maybe: Maybe[T]): T =
+  ## Returns the value of `maybe`. Raises ``FieldError`` if it is `nothing`.
+  if not maybe:
+    raise newException(FieldError, "Can't obtain a value from a `nothing`")
+  maybe.val
+
+
+template `or`*[T](maybe: Maybe[T], default: T): T =
+  ## Returns the value of `maybe`, or `default` if it is `nothing`.
+  if maybe: maybe.val
+  else: default
+
+template `or`*[T](a, b: Maybe[T]): Maybe[T] =
+  ## Returns `a` if it is `just`, otherwise `b`.
+  if a: a
+  else: b
+
+template `?=`*(into: expr, maybe: Maybe): bool =
+  ## Returns ``true`` if `maybe` isn't `nothing`.
+  ##
+  ## Injects a variable with the name specified by the argument `into`
+  ## with the value of `maybe`, or its type's default value if it is `nothing`.
+  ##
+  ## .. code-block:: nim
+  ##
+  ##   proc message(): ?string =
+  ##     just "Hello"
+  ##
+  ##   if m ?= message():
+  ##     echo m
+  var into {.inject.}: type(maybe.val)
+  if maybe:
+    into = maybe.val
+  maybe
+
+
+proc `==`*(a, b: Maybe): bool =
+  ## Returns ``true`` if both ``Maybe`` are `nothing`,
+  ## or if they have equal values
+  (a.has and b.has and a.val == b.val) or (not a.has and not b.has)
+
+proc `$`[T](maybe: Maybe[T]): string =
+  ## Converts to string: `"just(value)"` or `"nothing(type)"`
+  if maybe.has:
+    "just(" & $maybe.val & ")"
+  else:
+    "nothing(" & T.name & ")"
+
+
+when isMainModule:
+  template expect(E: expr, body: stmt) =
+    try:
+      body
+      assert false, E.type.name & " not raised"
+    except E:
+      discard
+
+
+  block: # example
+    proc find(haystack: string, needle: char): ?int =
+      for i, c in haystack:
+        if c == needle:
+          return just i
+
+    assert("abc".find('c')[] == 2)
+
+    let result = "team".find('i')
+
+    assert result == nothing(int)
+    assert result.has == false
+
+    if pos ?= "nim".find('i'):
+      assert pos is int
+      assert pos == 1
+    else:
+      assert false
+
+    assert(("team".find('i') or -1) == -1)
+    assert(("nim".find('i') or -1) == 1)
+
+  block: # just
+    assert just(6)[] == 6
+    assert just("a").unsafeVal == "a"
+    assert just(6).has
+    assert just("a")
+
+  block: # nothing
+    expect FieldError:
+      discard nothing(int)[]
+    assert(not nothing(int).has)
+    assert(not nothing(string))
+
+  block: # equality
+    assert just("a") == just("a")
+    assert just(7) != just(6)
+    assert just("a") != nothing(string)
+    assert nothing(int) == nothing(int)
+
+    when compiles(just("a") == just(5)):
+      assert false
+    when compiles(nothing(string) == nothing(int)):
+      assert false
+
+  block: # stringification
+    assert "just(7)" == $just(7)
+    assert "nothing(int)" == $nothing(int)
+
+  block: # or
+    assert just(1) or just(2) == just(1)
+    assert nothing(string) or just("a") == just("a")
+    assert nothing(int) or nothing(int) == nothing(int)
+    assert just(5) or 2 == 2
+    assert nothing(string) or "a" == "a"
+
+    when compiles(just(1) or "2"):
+      assert false
+    when compiles(nothing(int) or just("a")):
+      assert false
+
+  block: # extraction template
+    if a ?= just(5):
+      assert a == 5
+    else:
+      assert false
+
+    if b ?= nothing(string):
+      assert false