Reference: data types¶
This page gives reference information on Daml’s data types.
Builtin types¶
Table of builtin primitive types¶
Type  For  Example  Notes 

Int 
integers  1 , 1000000 , 1_000_000 
Int values are signed 64bit integers which represent numbers between 9,223,372,036,854,775,808 and 9,223,372,036,854,775,807 inclusive. Arithmetic operations raise an error on overflows and division by 0 . To make long numbers more readable you can optionally add underscores. 
Decimal 
short for Numeric 10 
1.0 
Decimal values are rational numbers with precision 38 and scale 10. 
Numeric n 
fixed point decimal numbers  1.0 
Numeric n values are rational numbers with up to 38 digits. The scale parameter n controls the number of digits after the decimal point, so for example, Numeric 10 values have 10 decimal places, and Numeric 20 values have 20 decimal places. The value of n must be between 0 and 37 inclusive. 
Text 
strings  "hello" 
Text values are strings of characters enclosed by double quotes. 
Bool 
boolean values  True , False 

Party 
unicode string representing a party  alice < getParty "Alice" 
Every party in a Daml system has a unique identifier of type Party . To create a value of type Party , use binding on the result of calling getParty . The party text can only contain alphanumeric characters,  , _ and spaces. 
Date 
models dates  date 2007 Apr 5 
To create a value of type Date , use the function date (to get this function, import DA.Date ). 
Time 
models absolute time (UTC)  time (date 2007 Apr 5) 14 30 05 
Time values have microsecond precision. To create a value of type Time , use a Date and the function time (to get this function, import DA.Time ). 
RelTime 
models differences between time values  seconds 1 , seconds (2) 
seconds 1 and seconds (2) represent the values for 1 and 2 seconds. There are no literals for RelTime . Instead they are created using one of days , hours , minutes and seconds (to get these functions, import DA.Time ). 
Escaping characters¶
Text
literals support backslash escapes to include their delimiter (\"
) and a backslash itself (\\
).
Time¶
Definition of time on the ledger is a property of the execution environment. Daml assumes there is a shared understanding of what time is among the stakeholders of contracts.
Lists¶
[a]
is the builtin data type for a list of elements of type a
. The empty list is denoted by []
and [1, 3, 2]
is an example of a list of type [Int]
.
You can also construct lists using []
(the empty
list) and ::
(which is an operator that appends an element to the front of a list). For example:
twoEquivalentListConstructions =
scenario do
assert ( [1, 2, 3] == 1 :: 2 :: 3 :: [] )
Records and record types¶
You declare a new record type using the data
and with
keyword:
data MyRecord = MyRecord
with
label1 : type1
label2 : type2
...
labelN : typeN
deriving (Eq, Show)
where:
label1
,label2
, …,labelN
are labels, which must be unique in the record typetype1
,type2
, …,typeN
are the types of the fields
There’s an alternative way to write record types:
data MyRecord = MyRecord { label1 : type1; label2 : type2; ...; labelN : typeN }
deriving (Eq, Show)
The format using with
and the format using { }
are exactly the same syntactically. The main difference is that when you use with
, you can use newlines and proper indentation to avoid the delimiting semicolons.
The deriving (Eq, Show)
ensures the data type can be compared (using ==
) and displayed (using show
). The line starting deriving
is required for data types used in fields of a template
.
In general, add the deriving
unless the data type contains function types (e.g. Int > Int
), which cannot be compared or shown.
For example:
 This is a record type with two fields, called first and second,
 both of type `Int`
data MyRecord = MyRecord with first : Int; second : Int
deriving (Eq, Show)
 An example value of this type is:
newRecord = MyRecord with first = 1; second = 2
 You can also write:
newRecord = MyRecord 1 2
Data constructors¶
You can use data
keyword to define a new data type, for example data Floor a = Floor a
for some type a
.
The first Floor
in the expression is the type constructor. The second Floor
is a data constructor that can be used to specify values of the Floor Int
type: for example, Floor 0
, Floor 1
.
In Daml, data constructors may take at most one argument.
An example of a data constructor with zero arguments is data Empty = Empty {}
. The only value of the Empty
type is Empty
.
Note
In data Confusing = Int
, the Int
is a data constructor with no arguments. It has nothing to do with the builtin Int
type.
Accessing record fields¶
To access the fields of a record type, use dot notation. For example:
 Access the value of the field `first`
val.first
 Access the value of the field `second`
val.second
Updating record fields¶
You can also use the with
keyword to create a new record on the basis of an existing replacing select fields.
For example:
myRecord = MyRecord with first = 1; second = 2
myRecord2 = myRecord with second = 5
produces the new record value MyRecord with first = 1; second = 5
.
If you have a variable with the same name as the label, Daml lets you use this without assigning it to make things look nicer:
 if you have a variable called `second` equal to 5
second = 5
 you could construct the same value as before with
myRecord2 = myRecord with second = second
 or with
myRecord3 = MyRecord with first = 1; second = second
 but Daml has a nicer way of putting this:
myRecord4 = MyRecord with first = 1; second
 or even
myRecord5 = r with second
Note
The with
keyword binds more strongly than function application. So for a function, say return
, either write return IntegerCoordinate with first = 1; second = 5
or return (IntegerCoordinate {first = 1; second = 5})
, where the latter expression is enclosed in parentheses.
Parameterized data types¶
Daml supports parameterized data types.
For example, to express a more general type for 2D coordinates:
 Here, a and b are type parameters.
 The Coordinate after the data keyword is a type constructor.
data Coordinate a b = Coordinate with first : a; second : b
An example of a type that can be constructed with Coordinate
is Coordinate Int Int
.
Type synonyms¶
To declare a synonym for a type, use the type
keyword.
For example:
type IntegerTuple = (Int, Int)
This makes IntegerTuple
and (Int, Int)
synonyms: they have the same type and can be used interchangeably.
You can use the type
keyword for any type, including Builtin types.
Function types¶
A function’s type includes its parameter and result types. A function foo
with two parameters has type ParamType1 > ParamType2 > ReturnType
.
Note that this can be treated as any other type. You could for instance give it a synonym using type FooType = ParamType1 > ParamType2 > ReturnType
.
Algebraic data types¶
An algebraic data type is a composite type: a type formed by a combination of other types. The enumeration data type is an example. This section introduces more powerful algebraic data types.
Product types¶
The following data constructor is not valid in Daml: data AlternativeCoordinate a b = AlternativeCoordinate a b
. This is because data constructors can only have one argument.
To get around this, wrap the values in a record:
data Coordinate a b = Coordinate {first: a; second: b}
.
These kinds of types are called product types.
A way of thinking about this is that the Coordinate Int Int
type has a first and second dimension (that is, a 2D product space). By adding an extra type to the record, you get a third dimension, and so on.
Sum types¶
Sum types capture the notion of being of one kind or another.
An example is the builtin data type Bool
. This is defined by data Bool = True  False deriving (Eq,Show)
, where True
and False
are data constructors with zero arguments . This means that a Bool
value is either True
or False
and cannot be instantiated with any other value.
Please note that all types which you intend to use as template or choice arguments need to derive at least from (Eq, Show).
A very useful sum type is data Optional a = None  Some a deriving (Eq,Show)
. It is part of
the Daml standard library.
Optional
captures the concept of a box, which can be empty or contain a value of type a
.
Optional
is a sum type constructor taking a type a
as parameter. It produces the sum type defined by the data constructors None
and Some
.
The Some
data constructor takes one argument, and it expects a value of type a
as a parameter.
Pattern matching¶
You can match a value to a specific pattern using the case
keyword.
The pattern is expressed with data constructors. For example, the Optional Int
sum type:
optionalIntegerToText (x : Optional Int) : Text =
case x of
None > "Box is empty"
Some val > "The content of the box is " <> show val
optionalIntegerToTextTest =
scenario do
let
x = Some 3
assert (optionalIntegerToText x == "The content of the box is 3")
In the optionalIntegerToText
function, the case
construct first tries to
match the x
argument against the None
data constructor, and in case of a match, the "Box is empty"
text is returned. In case of no match, a match is attempted for x
against the next pattern in the list, i.e., with the Some
data constructor. In case of a match, the content of the value attached to the Some
label is bound to the val
variable, which is then used in the corresponding output text string.
Note that all patterns in the case construct need to be complete, i.e., for each x
there must be at least one pattern that matches. The patterns are tested from top to bottom, and the expression for the first pattern that matches will be executed. Note that _
can be used as a catchall pattern.
You could also case distinguish a Bool
variable using the True
and False
data constructors and achieve the same behavior as an ifthenelse expression.
As an example, the following is an expression for a Text
:
let
l = [1, 2, 3]
in case l of
[] > "List is empty"
_ :: [] > "List has one element"
_ :: _ :: _ > "List has at least two elements"
Notice the use of nested pattern matching above.
Note
An underscore was used in place of a variable name. The reason for this is that Daml Studio produces a warning for all variables that are not being used. This is useful in detecting unused variables. You can suppress the warning by naming the variable with an initial underscore.