I am writing SML program to update records in a list.For example, I have type person_name.
type person_name = {fname:string, lname:string, mname:string}
Then I have person_bio which has person_name embedded in it.
type person_bio = {age:real, gender:string, name:person_name, status:string}
Next I have employee which has person_bio.
type employee = {p:person_bio, payrate:real, whours:real} list;
Now, I have to define function 'updateLastName' by passing the first name.
As of now, created one record 'e1' with below data.
{p={age=40.0,gender="M",name{fname="rob",lname="sen",mname=""},status="M"},
payrate=30.0,whours=10.0}
But I am facing challenge to traverse the list and then updating one field in record.
fun updateLastName(x:string,l:employee)=
if (L=[]) then []
else if (x= #fname(#name(#p hd l)) //cheking name of 1st record in list
//not getting how to update,this kind of line did not work
#fname(#name(#p hd l) = "abc"
else updateLastName(x,tl(l)); // hope this is right
Please suggest.
You have stumbled upon something difficult: Updating a deeply nested record.
For records you have getters, so #fname (#name (#p employee)) gets the field that you're checking against to know that this is the employee whose last name you are going to update. But records don't grant you equivalent setters, so you have to make those. If you're curious, lenses (Haskell) are a general way to solve this, but I don't know of any implementation of lenses for Standard ML.
I'll go ahead and remove the list part in your employee type; you should probably want an employee list if you want multiple employees modelled, rather than to say that an employee is multiple persons.
type person_name = { fname:string, lname:string, mname:string }
type person_bio = { age:real, gender:string, name:person_name, status:string }
type employee = { p:person_bio, payrate:real, whours:real }
val name1 = { fname = "John", lname = "Doe", mname = "W." } : person_name
val bio1 = { age = 42.0, gender = "M", name = name1, status = "?" } : person_bio
val my_employee1 = { p = bio1, payrate = 1000.0, whours = 37.0 } : employee
val name2 = { fname = "Freddy", lname = "Mercury", mname = "X." } : person_name
val bio2 = { age = 45.0, gender = "M", name = name2, status = "?" } : person_bio
val my_employee2 = { p = bio2, payrate = 2000.0, whours = 37.0 } : employee
val my_employees = [ my_employee1, my_employee2 ] : employee list
As for the setters (the ones that you could automatically derive using lenses),
fun setP (p : person_bio, e : employee) =
{ p = p
, payrate = #payrate e
, whours = #whours e } : employee
fun setName (name : person_name, pb : person_bio) =
{ age = #age pb
, gender = #gender pb
, name = name
, status = #status pb } : person_bio
fun setLname (lname, pn : person_name) =
{ fname = #fname pn
, lname = lname
, mname = #mname pn } : person_name
you can compose these, e.g. like:
- setP (setName (setLname ("Johnson", #name (#p my_employee1)), #p my_employee1), my_employee1)
> val it =
{p =
{age = 42.0, gender = "M",
name = {fname = "John", lname = "Johnson", mname = "W."},
status = "?"}, payrate = 1000.0, whours = 37.0} :
{p :
{age : real, gender : string,
name : {fname : string, lname : string, mname : string},
status : string}, payrate : real, whours : real}
Or you can split that line a little apart to make it more readable:
fun updateLname (fname, lname, employees) =
let fun update employee =
if #fname (#name (#p employee)) = fname
then let val new_name = setLname (lname, #name (#p employee))
val new_bio = setName (new_name, #p employee)
val new_employee = setP (new_bio, employee)
in new_employee end
else employee
in List.map update employees
end
Trying this out:
- updateLname ("Freddy", "Johnson", my_employees);
> val it =
[{p = ... {fname = "John", lname = "Doe", mname = "W."}, ... },
{p = ... {fname = "Freddy", lname = "Johnson", mname = "X."}, ... }]
- updateLname ("John", "Johnson", my_employees);
> val it =
[{p = ... {fname = "John", lname = "Johnson", mname = "W."}, ... },
{p = ... {fname = "Freddy", lname = "Mercury", mname = "X."}, ... }]