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057a18573a
Implements the new `module` header syntax as described in "module and package changes" [1]: ``` module [Request, Response, req] ``` The old syntax should still work fine, and is automatically upgraded to the new one when running `roc format`. [1] https://docs.google.com/document/d/1E_77fO-44BtoBtXoVeWyGh1xN2KRTWTu8q6i25RNNx0/edit
94 lines
3.2 KiB
Text
94 lines
3.2 KiB
Text
module [Result, isOk, isErr, map, mapErr, try, onErr, withDefault]
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import Bool exposing [Bool]
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## The result of an operation that could fail: either the operation went
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## okay, or else there was an error of some sort.
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Result ok err : [Ok ok, Err err]
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## Returns `Bool.true` if the result indicates a success, else returns `Bool.false`
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## ```roc
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## Result.isOk (Ok 5)
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## ```
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isOk : Result ok err -> Bool
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isOk = \result ->
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when result is
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Ok _ -> Bool.true
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Err _ -> Bool.false
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## Returns `Bool.true` if the result indicates a failure, else returns `Bool.false`
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## ```roc
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## Result.isErr (Err "uh oh")
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## ```
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isErr : Result ok err -> Bool
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isErr = \result ->
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when result is
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Ok _ -> Bool.false
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Err _ -> Bool.true
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## If the result is `Ok`, returns the value it holds. Otherwise, returns
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## the given default value.
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## ```roc
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## Result.withDefault (Ok 7) 42
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## Result.withDefault (Err "uh oh") 42
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## ```
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withDefault : Result ok err, ok -> ok
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withDefault = \result, default ->
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when result is
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Ok value -> value
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Err _ -> default
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## If the result is `Ok`, transforms the value it holds by running a conversion
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## function on it. Then returns a new `Ok` holding the transformed value. If the
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## result is `Err`, this has no effect. Use [mapErr] to transform an `Err`.
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## ```roc
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## Result.map (Ok 12) Num.neg
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## Result.map (Err "yipes!") Num.neg
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## ```
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##
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## Functions like `map` are common in Roc; see for example [List.map],
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## `Set.map`, and `Dict.map`.
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map : Result a err, (a -> b) -> Result b err
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map = \result, transform ->
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when result is
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Ok v -> Ok (transform v)
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Err e -> Err e
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## If the result is `Err`, transforms the value it holds by running a conversion
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## function on it. Then returns a new `Err` holding the transformed value. If
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## the result is `Ok`, this has no effect. Use [map] to transform an `Ok`.
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## ```roc
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## Result.mapErr (Err "yipes!") Str.isEmpty
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## Result.mapErr (Ok 12) Str.isEmpty
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## ```
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mapErr : Result ok a, (a -> b) -> Result ok b
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mapErr = \result, transform ->
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when result is
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Ok v -> Ok v
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Err e -> Err (transform e)
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## If the result is `Ok`, transforms the entire result by running a conversion
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## function on the value the `Ok` holds. Then returns that new result. If the
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## result is `Err`, this has no effect. Use `onErr` to transform an `Err`.
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## ```roc
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## Result.try (Ok -1) \num -> if num < 0 then Err "negative!" else Ok -num
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## Result.try (Err "yipes!") \num -> if num < 0 then Err "negative!" else Ok -num
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## ```
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try : Result a err, (a -> Result b err) -> Result b err
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try = \result, transform ->
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when result is
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Ok v -> transform v
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Err e -> Err e
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## If the result is `Err`, transforms the entire result by running a conversion
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## function on the value the `Err` holds. Then returns that new result. If the
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## result is `Ok`, this has no effect. Use `try` to transform an `Ok`.
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## ```roc
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## Result.onErr (Ok 10) \errorNum -> Str.toU64 errorNum
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## Result.onErr (Err "42") \errorNum -> Str.toU64 errorNum
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## ```
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onErr : Result a err, (err -> Result a otherErr) -> Result a otherErr
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onErr = \result, transform ->
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when result is
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Ok v -> Ok v
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Err e -> transform e
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