Dhall.Bash

This library exports two utilities for compiling Dhall expressions to Bash:

• dhallToExpression, which emits a Bash expression (i.e. a valid right-hand side for an assignment)
• dhallToStatement, which emits a Bash declare or unset statement suitable for use with eval

dhallToExpression only supports the conversion of primitive values, such as:

• Bool - which translates to a string that is either "true" or "false"
• Natural - which translates to a Bash integer
• Integer - which translates to a Bash integer
• Text - which translates to a Bash string (properly escaped if necessary)

The dhall-to-bash executable by default tries to compile Dhall expressions to Bash expressions using the dhallToExpression function. For example:

$ dhall-to-bash <<< 'True'
true
$ dhall-to-bash <<< 'False'
false
$ dhall-to-bash <<< '1'
1
$ dhall-to-bash <<< '+1'
1
$ dhall-to-bash <<< '"ABC"'
ABC
$ dhall-to-bash <<< '" X "'
$' X '
$ dhall-to-bash <<< 'Natural/even +100'
true

The output of dhallToExpression is a valid Bash expression that can be embedded anywhere Bash expressions are valid, such as the right-hand side of an assignment statement:

$ FOO=$(dhall-to-bash <<< 'List/length Natural [1, 2, 3]')
$ echo "${FOO}"
3

dhallToStatement supports a wider range of expressions by also adding support for:

• Optional - which translates to a variable which is either set or unset
• List - which translates to a Bash array
• records - which translate to Bash associative arrays

The dhall-to-bash executable can emit a statement instead of an expression if you add the --declare flag specifying which variable to set or unset. For example:

$ dhall-to-bash --declare FOO <<< 'None Natural'
unset FOO
$ dhall-to-bash --declare FOO <<< 'Some 1'
declare -r -i FOO=1
$ dhall-to-bash --declare FOO <<< 'Some (Some 1)'
declare -r -i FOO=1
$ dhall-to-bash --declare FOO <<< 'Some (None Natural)'
unset FOO
$ dhall-to-bash --declare FOO <<< '[1, 2, 3]'
declare -r -a FOO=(1 2 3)
$ dhall-to-bash --declare FOO <<< '{ bar = 1, baz = True }'
declare -r -A FOO=([bar]=1 [baz]=true)

The output of dhallToExpression is either a declare or unset Bash statement that you can pass to eval:

$ eval $(dhall-to-bash --declare FOO <<< '{ bar = 1, baz = True }')
$ echo "${FOO[bar]}"
1
$ echo "${FOO[baz]}"
true

dhall-to-bash declares variables read-only (i.e. -r) to prevent you from accidentally overwriting, deleting or mutating variables:

$ eval $(dist/build/dhall-to-bash/dhall-to-bash --declare BAR <<< '1')
$ echo "${BAR"}
1
$ unset BAR
bash: unset: BAR: cannot unset: readonly variable
$ eval $(dist/build/dhall-to-bash/dhall-to-bash --declare BAR <<< '2')
bash: declare: BAR: readonly variable

Dhall to Bash

dhallToExpression

Arguments

:: Expr s X

Dhall expression to compile

-> Either ExpressionError ByteString

Bash expression or compile failure

Compile a Dhall expression to a Bash expression

This only supports:

• Bools
• Naturals
• Integers
• Texts

dhallToStatement

Arguments

:: Expr s X

Dhall expression to compile

-> ByteString

Variable to declare or unset

-> Either StatementError ByteString

Bash statement or compile failure

Compile a Dhall expression to a Bash statement that declares or unsets a a variable of your choice

This only supports:

• Bools
• Naturals
• Integers
• Texts
• Optionals
• Lists
• records

Exceptions

data ExpressionError

This is the exception type for errors that might arise when translating Dhall expressions to Bash expressions

Because the majority of Dhall language features do not easily translate to Bash this just returns the expression that failed

Constructors

UnsupportedExpression (Expr X X)

Instances

Show ExpressionError

Defined in Dhall.Bash

Methods

showsPrec :: Int -> ExpressionError -> ShowS

show :: ExpressionError -> String

showList :: [ExpressionError] -> ShowS

Exception ExpressionError

Defined in Dhall.Bash

Methods

toException :: ExpressionError -> SomeException

fromException :: SomeException -> Maybe ExpressionError

displayException :: ExpressionError -> String

data StatementError

This is the exception type for errors that might arise when translating Dhall expressions to Bash statements

Because the majority of Dhall language features do not easily translate to Bash this just returns the expression that failed

Constructors

UnsupportedStatement (Expr X X)

UnsupportedSubexpression (Expr X X)

Instances

Show StatementError

Defined in Dhall.Bash

Methods

showsPrec :: Int -> StatementError -> ShowS

show :: StatementError -> String

showList :: [StatementError] -> ShowS

Exception StatementError

Defined in Dhall.Bash

Methods

toException :: StatementError -> SomeException

fromException :: SomeException -> Maybe StatementError

displayException :: StatementError -> String