From 41cfd04f64e10f42231079a79ea789b70e8d1b6c Mon Sep 17 00:00:00 2001
From: bch29 <bradleyhardy29@gmail.com>
Date: Tue, 15 Dec 2020 11:51:07 +0000
Subject: [PATCH] feat: Add `Display` typeclass

---
 std/display.glu | 22 ++++++++++++++++++++++
 std/float.glu   |  7 ++++++-
 std/int.glu     |  6 ++++++
 std/prelude.glu |  4 ++++
 std/string.glu  |  4 ++++
 5 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 std/display.glu

diff --git a/std/display.glu b/std/display.glu
new file mode 100644
index 0000000000..ed44cea1e7
--- /dev/null
+++ b/std/display.glu
@@ -0,0 +1,22 @@
+//@NO-IMPLICIT-PRELUDE
+//! Value to string conversion, intended for strings passed to users.
+
+/// `Display a` represents a conversion function from `a` to a pretty string.
+#[implicit]
+type Display a = { display : a -> String }
+
+/// Converts a value into a string.
+/// ```
+/// let { ? } = import! std.effect
+/// let { assert_eq, ? } = import! std.test
+///
+/// seq assert_eq (display 123) "123"
+/// seq assert_eq (display 3.14) "3.14"
+/// assert_eq (display "abc") "abc"
+/// ```
+let display ?d : [Display a] -> a -> String = d.display
+
+{
+    Display,
+    display,
+}
diff --git a/std/float.glu b/std/float.glu
index 35248f3c2b..762ecfa75f 100644
--- a/std/float.glu
+++ b/std/float.glu
@@ -1,7 +1,7 @@
 //@NO-IMPLICIT-PRELUDE
 //! The 64-bit floating point type.
 
-let { Semigroup, Monoid, Group, Eq, Ord, Ordering, Num, Show } = import! std.prelude
+let { Semigroup, Monoid, Group, Eq, Ord, Ordering, Num, Show, Display } = import! std.prelude
 
 let additive =
     let semigroup : Semigroup Float = { append = \x y -> x #Float+ y }
@@ -55,6 +55,10 @@ let show : Show Float = {
     show = (import! std.prim).show_float,
 }
 
+let display : Display Float = {
+    display = (import! std.prim).show_float,
+}
+
 {
     additive,
     multiplicative,
@@ -62,6 +66,7 @@ let show : Show Float = {
     ord,
     num,
     show,
+    display,
     ..
     import! std.float.prim
 }
diff --git a/std/int.glu b/std/int.glu
index 09affe7ac9..4ab81dcbdf 100644
--- a/std/int.glu
+++ b/std/int.glu
@@ -7,6 +7,7 @@ let { Group } = import! std.group
 let { Eq, Ord, Ordering } = import! std.cmp
 let { Num } = import! std.num
 let { Show } = import! std.show
+let { Display } = import! std.display
 
 let additive =
     let semigroup : Semigroup Int = {
@@ -59,6 +60,10 @@ let show : Show Int = {
     show = (import! std.prim).show_int,
 }
 
+let display : Display Int = {
+    display = (import! std.prim).show_int,
+}
+
 {
     additive,
     multiplicative,
@@ -66,6 +71,7 @@ let show : Show Int = {
     ord,
     num,
     show,
+    display,
     ..
     import! std.int.prim
 }
diff --git a/std/prelude.glu b/std/prelude.glu
index 5e584019fc..5553757b24 100644
--- a/std/prelude.glu
+++ b/std/prelude.glu
@@ -13,6 +13,7 @@ let { Monoid, empty } = import! std.monoid
 let { Group } = import! std.group
 let { Eq, Ord, Bool, Ordering, (==), (/=), (<), (<=), (>=), (>) } = import! std.cmp
 let { Show, show } = import! std.show
+let { Display, display } = import! std.display
 let { Category, id, compose } = import! std.category
 let { Num, (+), (-), (*), (/), negate } = import! std.num
 let { Bool, not } = import! std.bool
@@ -64,6 +65,9 @@ let { flat_map } = import! std.monad
     Show,
     show,
 
+    Display,
+    display,
+
     Option,
     Bool,
 
diff --git a/std/string.glu b/std/string.glu
index d26068f4d0..6d65cbab9b 100644
--- a/std/string.glu
+++ b/std/string.glu
@@ -6,6 +6,7 @@ let prim = import! std.prim
 let { Semigroup, (<>) } = import! std.semigroup
 let { Monoid } = import! std.monoid
 let { Show } = import! std.show
+let { Display } = import! std.display
 let { Eq, Ord, Ordering } = import! std.cmp
 let function = import! std.function
 
@@ -25,10 +26,13 @@ let ord : Ord String = { eq, compare = prim.string_compare }
 
 let show : Show String = { show = \s -> "\"" ++ s ++ "\"" }
 
+let display : Display String = { display = \s -> s }
+
 {
     eq,
     ord,
     show,
+    display,
     semigroup,
     monoid,
     (++),