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(;module: {#;doc "Exception-handling functionality built on top of the Result type."}
lux
(lux (control monad)
(data ["R" result]
[maybe]
[text "text/" Monoid<Text>])
[macro]
(macro [code]
["s" syntax #+ syntax: Syntax]
(syntax ["cs" common]
(common ["csr" reader]
["csw" writer])))))
## [Types]
(type: #export Exception
{#;doc "An exception provides a way to decorate error messages."}
(-> Text Text))
## [Values]
(def: #hidden _text/append_
(-> Text Text Text)
text/append)
(def: #export (match? exception error)
(-> Exception Text Bool)
(text;starts-with? (exception "") error))
(def: #export (catch exception then try)
{#;doc "If a particular exception is detected on a possibly-erroneous value, handle it.
If no exception was detected, or a different one from the one being checked, then pass along the original value."}
(All [a]
(-> Exception (-> Text a) (R;Result a)
(R;Result a)))
(case try
(#R;Success output)
(#R;Success output)
(#R;Error error)
(let [reference (exception "")]
(if (text;starts-with? reference error)
(#R;Success (|> error
(text;clip (text;size reference) (text;size error))
maybe;assume
then))
(#R;Error error)))))
(def: #export (otherwise to-do try)
{#;doc "If no handler could be found to catch the exception, then run a function as a last-resort measure."}
(All [a]
(-> (-> Text a) (R;Result a) a))
(case try
(#R;Success output)
output
(#R;Error error)
(to-do error)))
(def: #export (return value)
{#;doc "A way to lift normal values into the result-handling context."}
(All [a] (-> a (R;Result a)))
(#R;Success value))
(def: #export (throw exception message)
{#;doc "Decorate an error message with an Exception and lift it into the result-handling context."}
(All [a] (-> Exception Text (R;Result a)))
(#R;Error (exception message)))
(syntax: #export (exception: [_ex-lev csr;export] [name s;local-symbol])
{#;doc (doc "Define a new exception type."
"It moslty just serves as a way to tag error messages for later catching."
(exception: #export Some-Exception))}
(do @
[current-module macro;current-module-name
#let [descriptor ($_ text/append "{" current-module ";" name "}" "\n")
g!message (code;symbol ["" "message"])]]
(wrap (list (` (def: (~@ (csw;export _ex-lev)) ((~ (code;symbol ["" name])) (~ g!message))
Exception
(_text/append_ (~ (code;text descriptor)) (~ g!message))))))))
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