Safe Haskell | None |
---|---|
Language | Haskell2010 |
- pun :: QuasiQuoter
Documentation
>>>
:set -XQuasiQuotes -XViewPatterns
patterns
>>>
let y = Label :: Label "y"
>>>
let x = Label :: Label "x"
>>>
[pun| x y |] <- return (x .=. 3 .*. y .=. "hi" .*. emptyRecord)
>>>
print (x,y)
(3,"hi")
expressions
Compare with the standard way to construct records above
>>>
let x = 3; y = "hi"
>>>
[pun|x y|]
Record{x=3,y="hi"}
nesting
Nesting is supported. The idea is that variables inside
{ }
are in another record. More concretely:
[pun| ab@{ a b } y z c{d} |]
as a pattern, it will bindings from an original record x
,
if you interpret (.) as a left-associative field lookup (as it
is in other languages):
let ab = xab a = x.ab.a b = x.ab.b y = x.y z = x.z -- c is not bound d = x.c.d
as an expression, it creates a new record which needs the variables
ab a b y z d
in-scope. ab
needs to be a record, and if it has
fields called a
or b
they are overridden by the values of a
and b
which are in scope.
( )
parens mean the same thing as { }
, except the pattern match
restricts the fields in the record supplied to be exactly the ones
provided. In other words
[pun| (x _ y{}) |] = list -- desugars to something like: Record ((Tagged x :: Tagged "x" s1) `HCons` (Tagged _ :: Tagged t s2) `HCons` (Tagged _ :: Tagged "y" s3) `HCons` HNil) = list
Where the s1
and s2
are allowed to fit whatever is in the HList.
Note that this also introduces the familiar wild card pattern (_
),
and shows again how to ensure a label is present but not bind a variable
to it.
See also examples/pun.hs
. In {}
patterns, pun
can work with
Variant
too.
pun :: QuasiQuoter Source #
requires labels to be promoted strings (kind Symbol), as provided by
Data.HList.Label6 (ie. the label for foo is Label :: Label "foo"
),
or Data.HList.Labelable