feat(stdlib): env.getenv(name) — value of an env var (buf)

Iterates envp, returns the bytes after KEY= for a matching entry, empty
buf if unset. Lets Lake tools read config from the environment (used by
lake-house to locate the toolchain portably).

Author: morphqdd

std/env.lake

@@ -4,12 +4,16 @@
 // `rt_cstr_to_buf` copies each null-terminated entry into a Lake-owned
 // `buf`, returned as an `{atom buf}` tuple.
 
++std.bytes.{ at size }
++std.process.{ alloc_or_die }
+
 @rt(rt_argc_raw)
 @rt(rt_argv_raw)
 @rt(rt_envp_raw)
 @rt(rt_load_ptr_raw)
 @rt(rt_cstr_to_buf)
 @rt(rt_cstr_len)
+@rt(rt_copy_bytes)
 
 // Number of command-line arguments, including argv[0].
 pub argc is {
@@ -46,3 +50,77 @@ pub envp_at is {
 
 // For entry length, callers use `std.bytes.size(b)` (re-exporting it
 // here would collide — bug #97).
+
+// ── getenv (#97) ────────────────────────────────────────────────
+//
+// `getenv(name)` returns the value of environment variable `name` (the
+// bytes after `=`), or an empty buf when the variable is absent.  An
+// env var set to the empty string is reported as absent too — fine for
+// path-style lookups (LAKEC / LAKE_STDLIB) where empty == unset.
+
+// 1 iff the first `nn` bytes of `e` equal `name`.
+env_prefix_eq is {
+  e buf name buf nn i64 i i64 -> ret i64 {
+    when i >= nn {
+      true  -> { ret 1 }
+      false -> {
+        when at(e i) == at(name i) {
+          true  -> { self(e name nn i + 1) }
+          false -> { ret 0 }
+        }
+      }
+    }
+  }
+}
+
+// If `e` is `name=<value>`, return a fresh buf with `<value>`, else empty.
+env_match is {
+  e buf name buf -> ret buf {
+    let en = size(e)
+    let nn = size(name)
+    when en < nn + 1 {
+      true  -> { ret alloc_or_die(0) }
+      false -> {
+        let pm = env_prefix_eq(e name nn 0)
+        when pm == 1 {
+          true -> {
+            let eq = at(e nn)
+            when eq == 61 {
+              true -> {
+                let vlen = en - nn - 1
+                let out = alloc_or_die(vlen)
+                rt_copy_bytes(out 0 e nn + 1 vlen)
+                ret out
+              }
+              false -> { ret alloc_or_die(0) }
+            }
+          }
+          false -> { ret alloc_or_die(0) }
+        }
+      }
+    }
+  }
+}
+
+getenv_at is {
+  name buf i i64 -> ret buf {
+    let base = rt_envp_raw()
+    let p = rt_load_ptr_raw(base i)
+    when p == 0 {
+      true  -> { ret alloc_or_die(0) }
+      false -> {
+        let entry = rt_cstr_to_buf(p)
+        let val = env_match(entry.1 name)
+        when size(val) > 0 {
+          true  -> { ret val }
+          false -> { self(name i + 1) }
+        }
+      }
+    }
+  }
+}
+
+// Value of env var `name`, or an empty buf if unset / empty.
+pub getenv is {
+  name buf -> ret buf { ret getenv_at(name 0) }
+}