# LinearType and ContextManager **URL:** \ **Category:** Mojo\ **Created:** [July 6, 2025, 7:02pm UTC](https://forum.modular.com/t/lineartype-and-contextmanager/1910 "2025-07-06T19:02:33Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![rd4com](https://avatars.discourse-cdn.com/v4/letter/r/cdc98d/32.png) [@rd4com](https://forum.modular.com/u/rd4com)\ **Post date:** [July 6, 2025, 7:02pm UTC](https://forum.modular.com/t/lineartype-and-contextmanager/1910/1 "2025-07-06T19:02:33Z")
Hello, i’d like to share an way to create an context-manager that uses LinearTypes. Also would appreciate any feedbacks from team on this. (ContextManager and requiring ` __copyinit__ ` or not) This struct greatly improved the API of an UI i’m working on (by a lot), while retaining all the benefits that LinearTypes brings. Example with emojis: ```mojo #✏️ Note: the struct have no `Movable` `Copyable` ! struct MoveCursor[ O:MutableOrigin=MutableOrigin.empty, border:StyledBorder = __StyleBorderNone, border_color: Fg= Fg.default ](): var ui: Pointer[UI, O] var m: StartedMeasurment[O] #LinearType ⚪ var after: Bool #Switcher 🔀 var storage_border: Border[O] #LinearType ⬜ @staticmethod fn AfterThis[border:StyledBorder = __StyleBorderNone, border_color: Fg= Fg.default]( mut ui:UI, out ret: MoveCursor[__origin_of(ui), border, border_color] ): ret = __type_of(ret)(ui) ret.after=True #Set switch to after ➡️ @staticmethod fn BelowThis[border:StyledBorder = __StyleBorderNone, border_color: Fg= Fg.default]( mut ui:UI, out ret: MoveCursor[__origin_of(ui), border, border_color] ): ret = __type_of(ret)(ui) ret.after=False #Set switch to below ⬇️ fn __init__ (out self, ref[O]ui:UI): self.ui = Pointer(to=ui) self.m = ui.start_measuring() self.after = False #Default switch to ➡️ @parameter if Self.__has_border[border](): #Initialize LinearType 🟩 for using it ! self.storage_border = Border[O](self.ui[]) else: #MarkInitialize LinearType 🟦 for __disable_del() later #Line is not here to not encourage unsafe things #Users can just initialize the type with an "Do nothing" parameter @staticmethod fn __has_border[b:StyledBorder]()->Bool: #If type is "none", return False (types-state) return not _type_is_eq[b, __StyleBorderNone]() fn __enter__ (mut self): ... #Nothing here, struct uses ` __init__ ` fn __exit__ (mut self): ... #Nothing here, struct uses ` __del__ ` fn __del__ (owned self): @parameter if Self.__has_border[border](): #Consume 1 🟩 with struct method that does something self.storage_border^.end_border[border](self.ui[], border_color) else: #Consume 2 🟦 with struct method that __disable_del self.storage_border^.__unsafe_del() # ⚪ Consume type into another LinearType (`CompletedMeasurement`) var tmp = self.m^.stop_measuring() if self.after: #Check the switch 🔀 tmp^.move_cursor_after() #Consume 1 🟢 else: tmp^.move_cursor_below() #Consume 2 🔵 ``` The result is this API: ```mojo with MoveCursor.BelowThis[StyledBorderSimple](ui): ... with MoveCursor.BelowThis[StyledBorderSimple, Fg.magenta](ui): ... with MoveCursor.BelowThis(ui): ... with MoveCursor.AfterThis[StyledBorderDouble](ui): ... with MoveCursor.AfterThis[StyledBorderDouble, Fg.magenta](ui): ... with MoveCursor.AfterThis(ui): ... ``` And compared to the wrapped API: ```mojo var m = ui.start_measuring() var b = m.start_border() #b now have origin of m "Hello world" in ui b^.end_border[StyledBorderSimple, Fg.blue]() #b end before m var m_stop = m^.stop_measuring() m_stop^.move_cursor_below() #all measured, can move draw cursor! ``` Complete example: > > > It is an struct that currently does 3 things: > \- start and stop measuring > \- move …cursor below or after > \- integrate optional border with style and colors > > See example: > \`📂 wip\_api\_playground/context\_managers.mojo\` ---
**Author:** ![melodyogonna](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/melodyogonna/32/33_2.png) [@melodyogonna](https://forum.modular.com/u/melodyogonna)\ **Post date:** [July 6, 2025, 7:29pm UTC](https://forum.modular.com/t/lineartype-and-contextmanager/1910/2 "2025-07-06T19:29:47Z")
Your `MovCursor` isn’t annotated to be a Linear type, so instances of it will be deallocated even if they’re not used - breaking the only rule of Linearity: `Types MUST be used exactly once`. ---
**Author:** ![rd4com](https://avatars.discourse-cdn.com/v4/letter/r/cdc98d/32.png) [@rd4com](https://forum.modular.com/u/rd4com)\ **Post date:** [July 6, 2025, 9:05pm UTC](https://forum.modular.com/t/lineartype-and-contextmanager/1910/3 "2025-07-06T21:05:29Z")
Good thinking, linear types are used only once, and wait, here is the twist 😃 Type instances must be consumed only once, and in an specific way, either automatically (like this custom ` __del__ `, called by asap), or manually 👍 This ` __del__ ` handles two ways of consuming one type for example. (Does not leave any of the instances stored in any fields unconsumed) Because the ` __del__ ` knows how to dispose of the types, in all branches, asap can do the cleanup, so i have to think this is correct. One cool thing is that this is an ui, so if there was a bug, the ui screen would look like mr potato from the movies lol. In the next example, if ` __del__ ` is removed/commented, mojo would have say, hey this lineartype is not del, let’s not build this, which is great: ```mojo @explicit_destroy struct MyLinear: var x: Int fn __init__ (out self): self.x=1 fn remove(owned self): __disable_del(self) struct NotLinear: #⬅️ Not an linear type var val: MyLinear fn __init__ (out self): self.val=MyLinear() fn __del__ (owned self): self.val^.remove() #⬅️ Try comment this fn main(): var value = NotLinear() print(value.val.x) ``` Sorry if this is a long response, i could be wrong and i appreciate your thinking @melodyogonna ! 🤝 (please double check this) ---
**Author:** ![melodyogonna](https://sea1.discourse-cdn.com/flex001/user_avatar/forum.modular.com/melodyogonna/32/33_2.png) [@melodyogonna](https://forum.modular.com/u/melodyogonna)\ **Post date:** [July 7, 2025, 7:45am UTC](https://forum.modular.com/t/lineartype-and-contextmanager/1910/4 "2025-07-07T07:45:36Z")
I’m thinking about what happens if I initialize movecursor without doing anything with it, like this: ```python fn main(): var cursor = MoveCursor() ``` Your ` __del__ ` assumes I will always call belowThis or AfterThis, but nothing compels me to, so your logic has a soundness problem. ---
**Author:** ![rd4com](https://avatars.discourse-cdn.com/v4/letter/r/cdc98d/32.png) [@rd4com](https://forum.modular.com/u/rd4com)\ **Post date:** [July 7, 2025, 10:19am UTC](https://forum.modular.com/t/lineartype-and-contextmanager/1910/5 "2025-07-07T10:19:43Z")
> [@melodyogonna](#): > > `var cursor = MoveCursor()` Huge thanks, yes that is an very valid observation, and it comes with a new feature! on a side note, you could not break the ui by going ahead and doing this: (but humbly it is work in progress, so it is far from ‘bug-free’) ```mojo var cursor = MoveCursor(ui) "hello" in ui "dear" in ui cursor^. __del__ () "world" in ui ``` ```mojo "Hello" in ui for _ in range(100): MoveCursor(ui) "World" in ui # output: # Hello # World ``` It would move the cursor below, which is the default ui behaviour anyway when not measuring 😃 (Also what happens when the measured area is [0,0] (width, height)) ➡ Again thanks for this lol, it means we could make it movable for recursivity too and enter when needed! I’d like to not entangle the ui too much in this thread tho, because this thread is really about LinearType and ContextManager, but that is really interesting, would you like to dive in the repo together sometime !? ---
**Author:** ![system](https://us1.discourse-cdn.com/flex001/uploads/modular/original/1X/2751e0fbdc595a99718b216730957e9db4448cfd.jpeg) [@system](https://forum.modular.com/u/system)\ **Post date:** [January 3, 2026, 10:20am UTC](https://forum.modular.com/t/lineartype-and-contextmanager/1910/6 "2026-01-03T10:20:17Z")
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