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outcome
outcome
Functions
converts outcome to a string
returns the result of T.as_string for a successful outcome, or
"--$e--" for e error.
returns the result of T.as_string for a successful outcome, or
"--$e--" for e error.
convert this outcome to an option
if the outcome is an error, the option is empty
if the outcome is an error, the option is empty
error of an outcome that is known to contain an error
This can only be called in cases where it is known for sure that this
outcome is an error. A runtime error will be created otherwise.
This can only be called in cases where it is known for sure that this
outcome is an error. A runtime error will be created otherwise.
value of an outcome that is known to contain a value
This can only be called in cases where it is known for sure that this
outcomee is not an error. A runtime error will be created otherwise.
This can only be called in cases where it is known for sure that this
outcomee is not an error. A runtime error will be created otherwise.
value of an outcome or default if outcome contains err
monadic operator
monadic operator
This enables a very idiomatic way of error handling: assume you are trying
to read 42 bytes from a file into a string. Let open be a feature that takes
a file name and returns an outcome of some abstract file descriptor, let read
be a feature that takes an abstract file descriptor and a number of bytes to
read and returns an outcome of a byte (u8) array. We want to read 42 bytes
from a file called "example.txt" into a string, and we wrap this into an outcome,
for handling the case that an error occurs when opening the file (i.e. it does
not exist) or when reading the file (i.e. example.txt is not actually a file but
a directory). Using match expressions, this is very cumbersome:
The monadic operator turns this into:
This enables a very idiomatic way of error handling: assume you are trying
to read 42 bytes from a file into a string. Let open be a feature that takes
a file name and returns an outcome of some abstract file descriptor, let read
be a feature that takes an abstract file descriptor and a number of bytes to
read and returns an outcome of a byte (u8) array. We want to read 42 bytes
from a file called "example.txt" into a string, and we wrap this into an outcome,
for handling the case that an error occurs when opening the file (i.e. it does
not exist) or when reading the file (i.e. example.txt is not actually a file but
a directory). Using match expressions, this is very cumbersome:
The monadic operator turns this into:
short-hand postfix operator for 'ok'
short-hand postfix operator for 'is_error'
returns o if outcome is ok, otherwise return the outcome's own
error.
error.
returns o if outcome is not ok, otherwise return this outcome's value.
Type Features
return function
outcome is a choice type that represents the result of a routine that
may either produce something useful or fail producing an error condition.
Several error conditions are needed if there are several very different
reasons for an operation to fail, e.g.
get_data (u User, t Type) outcome data IO_Error Permission_Error is
read_file t Type outcome data IO_Error is
Note that 'outcome data IO_Error' is not assignment compatible with
'outcome data IO_Error Permission_Error', it has to be unwrapped first.
This unwrapping, however, requires very little boilerplate, it is done
using the '?' on the result of the call to 'read_file': This unwraps
'outcome data IO_Error' into 'IO_Error', which would be returned abruptly,
and 'data', which would be returned normally. Both are assignment
compatible to 'outcome data IO_Error Permission_Error', so everything
is fine.