Stream: Beginner Questions

Topic: Why does map_t function fails

Hongjian Jiang (Jul 25 2023 at 13:08):

datatype (atoms: 'a) rexp =
  is_Zero: Zero |
  is_One: One |
  Symbol "'a ⇒ bool" |
  Plus "('a rexp)" "('a rexp)" |
  Times "('a rexp)" "('a rexp)" |
  Star "('a rexp)"

I have defined regular expression type, but I found the function of map_rexp, rexp.map_compare both failed.

Why does this happen, how to fix it.

Mathias Fleury (Jul 25 2023 at 13:18):

And I am sure that you tried your example, reduced it (by removing branches) until map was generated. So you realized that the problem is Symbol "'a ⇒ bool".

Mathias Fleury (Jul 25 2023 at 13:18):

So what was the question?

Hongjian Jiang (Jul 25 2023 at 13:21):

My initial idea is the Symbol means a predicate, take any input, then return a boolean value.

Hongjian Jiang (Jul 25 2023 at 13:21):

Is that such definition not work in the datatype context

Mathias Fleury (Jul 25 2023 at 13:22):

how would you define it?

Mathias Fleury (Jul 25 2023 at 13:23):

Your map has type ('a ⇒ 'b) ⇒ 'a rexp ⇒ 'b rexp

Mathias Fleury (Jul 25 2023 at 13:23):

how would you convert a function 'a ⇒ bool?

Mathias Fleury (Jul 25 2023 at 13:23):

This is a real question, I have absolutely no clue

Mathias Fleury (Jul 25 2023 at 13:24):

if the type was ('a ⇒ 'a) ⇒ 'a rexp ⇒ 'a rexp then I can image function composition in Symbol, but not here

Hongjian Jiang (Jul 25 2023 at 13:26):

I have seen an example datatype ('a, 'b) ite = ITE "'a ⇒ bool" "'a ⇒ 'b" "'a ⇒ 'b" here

Hongjian Jiang (Jul 25 2023 at 13:27):

it takes a predicate in the condition

Hongjian Jiang (Jul 25 2023 at 13:27):

Sorry for unfamilr with the type function

Hongjian Jiang (Jul 25 2023 at 13:30):

maybe this would work, if you have any suggestion.

datatype (atoms:'a, 'b) rexp =
  is_Zero: Zero |
  is_One: One |
  Symbol "'a ⇒ bool" |
  Plus "('a, 'b) rexp" "('a, 'b) rexp" |
  Times "('a, 'b) rexp""('a, 'b) rexp" |
  Star "('a, 'b) rexp"

Mathias Fleury (Jul 25 2023 at 13:34):

Then you should report this to the mailing list…

Mathias Fleury (Jul 25 2023 at 13:35):

maybe @Martin Desharnais or @Dmitriy Traytel know why adding a second type parameter makes the map function work suddenly

Simon Roßkopf (Jul 25 2023 at 16:55):

I do not think it is possible to generate such a map function. As Mathias said: How would such a function look.

Note that the map function for your ITE example only maps over the return types of the functions (the 'bs). There is no mapping over the 'as happening there

Mathias Fleury (Jul 25 2023 at 17:26):

that what I though too, but for the second version with the 'b it works…

Mathias Fleury (Jul 25 2023 at 17:26):

(but I did not check how the definition looks like)

Simon Roßkopf (Jul 25 2023 at 17:43):

It does generate a map function, but only one mapping over the second type argument.

"map_rexp" :: "('a ⇒ 'b) ⇒ ('c, 'a) rexp ⇒ ('c, 'b) rexp

Last updated: Nov 13 2025 at 04:27 UTC